Taboo Topic<\/h1>\r\n<\/div>\r\nThe banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.\r\n\r\nWhat Is the Menstrual Cycle?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary]<\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0[pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a [pb_glossary id=\"5253\"]follicle[\/pb_glossary], ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.\r\nMenarche and Menopause<\/h2>\r\nThe female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0[pb_glossary id=\"5300\"]menarche[\/pb_glossary].<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.\r\n\r\nThe cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0[pb_glossary id=\"5255\"]menopause[\/pb_glossary].<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.\r\nVariation in the Menstrual Cycle<\/h2>\r\nThe length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.\r\n\r\nOvarian Cycle<\/h1>\r\n<\/div>\r\nThe events of the menstrual cycle that take place in the ovaries make up the\u00a0[pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary].<\/strong> It consists of changes that occur in the [pb_glossary id=\"5253\"]follicles[\/pb_glossary] of one of the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary]. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.\r\n\r\n[caption id=\"attachment_5302\" align=\"aligncenter\" width=\"919\"]![\"18.8.2](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ovarian-Cycle-e1593185427416-2.png\")
Figure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em>[\/caption]\r\nFollicular Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5303\"]follicular phase[\/pb_glossary]<\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several [pb_glossary id=\"5253\"]ovarian follicles<\/span>[\/pb_glossary]<\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of [pb_glossary id=\"5206\"]follicle stimulating hormone[\/pb_glossary] (FSH), which is secreted by the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. The maturing follicle releases [pb_glossary id=\"5995\"]estrogen[\/pb_glossary], the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span>\r\n\r\n[caption id=\"attachment_5304\" align=\"aligncenter\" width=\"724\"]![\"18.8.3](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Figure_43_04_04-e1593185848433-2.png\")
Figure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em>[\/caption]\r\nOvulation<\/h2>\r\n[pb_glossary id=\"5239\"]Ovulation[\/pb_glossary]<\/strong>\u00a0is the second phase of the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of [pb_glossary id=\"5205\"]luteinizing hormone[\/pb_glossary] (LH) from the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary], the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary [pb_glossary id=\"5274\"]oocyte[\/pb_glossary].\r\nLuteal Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5305\"]luteal phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, [pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary] cause the Graafian follicle that ovulated the egg to transform into a structure called a [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary]. The corpus luteum secretes [pb_glossary id=\"3411\"]progesterone[\/pb_glossary], which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.\r\n\r\n \t- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\r\n \t
- If\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0has occurred so a [pb_glossary id=\"2471\"]zygote[\/pb_glossary] forms and then divides to become a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary], the outer layer of the blastocyst produces a\u00a0hormone\u00a0called [pb_glossary id=\"5307\"]human chorionic gonadotropin[\/pb_glossary] (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\r\n<\/ul>\r\n\r\n
Uterine Cycle<\/h1>\r\n<\/div>\r\nThe events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the uterus make up the\u00a0[pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]<\/strong>. This cycle consists of changes that occur mainly in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary], which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.\r\n\r\n[caption id=\"attachment_5309\" align=\"aligncenter\" width=\"488\"]![\"18.8.4](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/Ovarian-and-menstrual-cycle-e1593188761715-2.jpg\")
Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em>[\/caption]\r\nMenstruation<\/h2>\r\n[pb_glossary id=\"5310\"]Menstruation[\/pb_glossary]<\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] has not taken place during the preceding menstrual cycle. During menstruation, the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] of the uterus, which has built up during the preceding cycle, degenerates and is shed from the [pb_glossary id=\"5102\"]uterus[\/pb_glossary], flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.\r\nProliferative Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary]<\/strong>\u00a0is the second phase of the uterine cycle. During this phase, [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] secreted by\u00a0cells\u00a0of the maturing [pb_glossary id=\"5253\"]ovarian follicle[\/pb_glossary] causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the [pb_glossary id=\"5235\"]cervix[\/pb_glossary] of the uterus to secrete larger amounts of thinner mucus that can help\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]\u00a0swim through the cervix and into the uterus, making fertilization more likely.\r\nSecretory Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5312\"]secretory phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the [pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]. During this phase, [pb_glossary id=\"3411\"]progesterone[\/pb_glossary] produced by the [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary] in the ovary stimulates further changes in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] so it is more receptive to implantation of a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary]. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] tissue in the uterine wall.\r\n\r\n \r\nBringing it All Together<\/h1>\r\nIt is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.\r\n\r\n[caption id=\"attachment_5313\" align=\"aligncenter\" width=\"1000\"]![\"18.8.8](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2023\/10\/1000px-MenstrualCycle2_en.svg_-2.png\")
Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em>[\/caption]\r\n\r\n \r\n\r\n\r\n18.8 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary] and uterus, and is controlled by [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary] hormones ([pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary]) and ovarian hormones ([pb_glossary id=\"5995\"]estrogen[\/pb_glossary] and [pb_glossary id=\"3411\"]progesterone[\/pb_glossary]).<\/li>\r\n \t
- The female reproductive period is delineated by [pb_glossary id=\"5300\"]menarche[\/pb_glossary], or the first menstrual period, which usually occurs around age 12 or 13; and by [pb_glossary id=\"5255\"]menopause[\/pb_glossary], or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\r\n \t
- The events of the menstrual cycle that take place in the ovaries make up the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It includes the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary] (when a [pb_glossary id=\"5253\"]follicle[\/pb_glossary] and its ovum mature due to rising levels of FSH), [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] (when the [pb_glossary id=\"5099\"]ovum[\/pb_glossary] is released from the ovary due to a rise in estrogen and a surge in LH), and the [pb_glossary id=\"5305\"]luteal phase[\/pb_glossary] (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\r\n \t
- The events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] make up the uterine cycle. It includes [pb_glossary id=\"5310\"]menstruation[\/pb_glossary], which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the [pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary], during which the endometrium builds up again until [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] occurs; and the [pb_glossary id=\"5312\"]secretory phase[\/pb_glossary], which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an [pb_glossary id=\"5262\"]embryo[\/pb_glossary].<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"650\"]<\/li>\r\n \t
- What is the menstrual cycle? Why is the menstrual cycle necessary in order for pregnancy to occur?<\/li>\r\n \t
- What organs are involved in the menstrual cycle?<\/li>\r\n \t
- Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?<\/li>\r\n \t
- Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.<\/li>\r\n \t
- If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.<\/li>\r\n \t
- Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/cjbgZwgdY7Q\r\nWhy do women have periods? TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/5B3Abpv0ysM\r\n
Girl's Rite of Passage | National Geographic, 2007.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 18.8.1<\/strong>\r\n\r\nWaterforPeople_Uganda<\/a> by WaterforPeople_Uganda<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 18.8.2<\/strong>\r\n\r\nOvarian Cycle<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used and adapted under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 18.8.3<\/strong>\r\n\r\nFigure_43_04_04<\/a>\u00a0by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license. (Original: modification of work by Mikael H\u00e4ggstr\u00f6m)\r\n\r\nFigure 18.8.4<\/strong>\r\n\r\nOvarian and menstrual cycle<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0 CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 18.8.5<\/strong>\r\n\r\n1000px-MenstrualCycle2_en.svg<\/a> by Isometrik<\/a> on Wikimedia Common is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span>27.15<\/span>\u00a0<\/span>Hormone levels in ovarian and menstrual cycles<\/span> [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/p>\r\nNational Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=5B3Abpv0ysM&feature=youtu.be<\/p>\r\n
OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In Open Stax, Biology<\/em> (Section 43.4). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction<\/p>\r\nTED-Ed. (2015, October 19). Why do women have periods? YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=cjbgZwgdY7Q&feature=youtu.be<\/p>","rendered":"
<\/p>\n![\"18.8.1](\"https:\/\/pressbooks.ccconline.org\/acchumanbio\/wp-content\/uploads\/sites\/152\/2019\/06\/WaterforPeople_Uganda-2.jpg\")
Figure 18.8.1 It’s healthy to talk about menstruation.<\/em><\/figcaption><\/figure>\n\nTaboo Topic<\/h1>\n<\/div>\n
The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.<\/p>\n
\nWhat Is the Menstrual Cycle?<\/h1>\n<\/div>\n
The\u00a0menstrual cycle<\/a><\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up endometrium<\/a> lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0pregnancy<\/a>\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a follicle<\/a>, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.<\/p>\nMenarche and Menopause<\/h2>\n
The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0menarche<\/a>.<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.<\/p>\nThe cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0menopause<\/a>.<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.<\/p>\nVariation in the Menstrual Cycle<\/h2>\n
The length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.<\/p>\n
\nOvarian Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle that take place in the ovaries make up the\u00a0ovarian cycle<\/a>.<\/strong> It consists of changes that occur in the follicles<\/a> of one of the ovaries<\/a>. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.<\/p>\nFigure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em><\/figcaption><\/figure>\nFollicular Phase<\/h2>\n
The\u00a0follicular phase<\/a><\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles<\/span><\/a><\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle stimulating hormone<\/a> (FSH), which is secreted by the pituitary gland<\/a>. The maturing follicle releases estrogen<\/a>, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span><\/p>\nFigure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em><\/figcaption><\/figure>\nOvulation<\/h2>\n
Ovulation<\/a><\/strong>\u00a0is the second phase of the ovarian cycle<\/a>. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone<\/a> (LH) from the pituitary gland<\/a>. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase<\/a>, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen<\/a> stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte<\/a>.<\/p>\nLuteal Phase<\/h2>\n
The\u00a0luteal phase<\/a><\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH<\/a> and LH<\/a> cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum<\/a>. The corpus luteum secretes progesterone<\/a>, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the endometrium<\/a> in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.<\/p>\n\n- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\n
- If\u00a0fertilization<\/a>\u00a0has occurred so a zygote<\/a> forms and then divides to become a blastocyst<\/a>, the outer layer of the blastocyst produces a\u00a0hormone\u00a0called human chorionic gonadotropin<\/a> (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\n<\/ul>\n\n
Uterine Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle<\/a> that take place in the uterus make up the\u00a0uterine cycle<\/a><\/strong>. This cycle consists of changes that occur mainly in the endometrium<\/a>, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.<\/p>\nFigure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em><\/figcaption><\/figure>\nMenstruation<\/h2>\n
Menstruation<\/a><\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization<\/a> has not taken place during the preceding menstrual cycle. During menstruation, the endometrium<\/a> of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus<\/a>, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.<\/p>\nProliferative Phase<\/h2>\n
The\u00a0proliferative phase<\/a><\/strong>\u00a0is the second phase of the uterine cycle. During this phase, estrogen<\/a> secreted by\u00a0cells\u00a0of the maturing ovarian follicle<\/a> causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix<\/a> of the uterus to secrete larger amounts of thinner mucus that can help\u00a0sperm<\/a>\u00a0swim through the cervix and into the uterus, making fertilization more likely.<\/p>\nSecretory Phase<\/h2>\n
The\u00a0secretory phase<\/a><\/strong>\u00a0is the third and final phase of the uterine cycle<\/a>. During this phase, progesterone<\/a> produced by the corpus luteum<\/a> in the ovary stimulates further changes in the endometrium<\/a> so it is more receptive to implantation of a blastocyst<\/a>. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle<\/a> tissue in the uterine wall.<\/p>\n <\/p>\n
Bringing it All Together<\/h1>\n
It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.<\/p>\nFigure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em><\/figcaption><\/figure>\n <\/p>\n
\n\n\n18.8 Summary<\/span><\/h1>\n<\/header>\n\n\n- The menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries<\/a> and uterus, and is controlled by pituitary gland<\/a> hormones (FSH<\/a> and LH<\/a>) and ovarian hormones (estrogen<\/a> and progesterone<\/a>).<\/li>\n
- The female reproductive period is delineated by menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n
- The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle<\/a>. It includes the follicular phase<\/a> (when a follicle<\/a> and its ovum mature due to rising levels of FSH), ovulation<\/a> (when the ovum<\/a> is released from the ovary due to a rise in estrogen and a surge in LH), and the luteal phase<\/a> (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\n
- The events of the menstrual cycle<\/a> that take place in the uterus<\/a> make up the uterine cycle. It includes menstruation<\/a>, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase<\/a>, during which the endometrium builds up again until ovulation<\/a> occurs; and the secretory phase<\/a>, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n18.8 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
What Is the Menstrual Cycle?<\/h1>\r\n<\/div>\r\nThe\u00a0[pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary]<\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0[pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a [pb_glossary id=\"5253\"]follicle[\/pb_glossary], ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.\r\nMenarche and Menopause<\/h2>\r\nThe female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0[pb_glossary id=\"5300\"]menarche[\/pb_glossary].<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.\r\n\r\nThe cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0[pb_glossary id=\"5255\"]menopause[\/pb_glossary].<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.\r\nVariation in the Menstrual Cycle<\/h2>\r\nThe length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.\r\n\r\nOvarian Cycle<\/h1>\r\n<\/div>\r\nThe events of the menstrual cycle that take place in the ovaries make up the\u00a0[pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary].<\/strong> It consists of changes that occur in the [pb_glossary id=\"5253\"]follicles[\/pb_glossary] of one of the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary]. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.\r\n\r\n[caption id=\"attachment_5302\" align=\"aligncenter\" width=\"919\"] Figure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em>[\/caption]\r\nFollicular Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5303\"]follicular phase[\/pb_glossary]<\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several [pb_glossary id=\"5253\"]ovarian follicles<\/span>[\/pb_glossary]<\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of [pb_glossary id=\"5206\"]follicle stimulating hormone[\/pb_glossary] (FSH), which is secreted by the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. The maturing follicle releases [pb_glossary id=\"5995\"]estrogen[\/pb_glossary], the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span>\r\n\r\n[caption id=\"attachment_5304\" align=\"aligncenter\" width=\"724\"] Figure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em>[\/caption]\r\nOvulation<\/h2>\r\n[pb_glossary id=\"5239\"]Ovulation[\/pb_glossary]<\/strong>\u00a0is the second phase of the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of [pb_glossary id=\"5205\"]luteinizing hormone[\/pb_glossary] (LH) from the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary], the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary [pb_glossary id=\"5274\"]oocyte[\/pb_glossary].\r\nLuteal Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5305\"]luteal phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, [pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary] cause the Graafian follicle that ovulated the egg to transform into a structure called a [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary]. The corpus luteum secretes [pb_glossary id=\"3411\"]progesterone[\/pb_glossary], which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.\r\n\r\n \t- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\r\n \t
- If\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0has occurred so a [pb_glossary id=\"2471\"]zygote[\/pb_glossary] forms and then divides to become a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary], the outer layer of the blastocyst produces a\u00a0hormone\u00a0called [pb_glossary id=\"5307\"]human chorionic gonadotropin[\/pb_glossary] (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\r\n<\/ul>\r\n\r\n
Uterine Cycle<\/h1>\r\n<\/div>\r\nThe events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the uterus make up the\u00a0[pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]<\/strong>. This cycle consists of changes that occur mainly in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary], which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.\r\n\r\n[caption id=\"attachment_5309\" align=\"aligncenter\" width=\"488\"] Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em>[\/caption]\r\nMenstruation<\/h2>\r\n[pb_glossary id=\"5310\"]Menstruation[\/pb_glossary]<\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] has not taken place during the preceding menstrual cycle. During menstruation, the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] of the uterus, which has built up during the preceding cycle, degenerates and is shed from the [pb_glossary id=\"5102\"]uterus[\/pb_glossary], flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.\r\nProliferative Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary]<\/strong>\u00a0is the second phase of the uterine cycle. During this phase, [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] secreted by\u00a0cells\u00a0of the maturing [pb_glossary id=\"5253\"]ovarian follicle[\/pb_glossary] causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the [pb_glossary id=\"5235\"]cervix[\/pb_glossary] of the uterus to secrete larger amounts of thinner mucus that can help\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]\u00a0swim through the cervix and into the uterus, making fertilization more likely.\r\nSecretory Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5312\"]secretory phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the [pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]. During this phase, [pb_glossary id=\"3411\"]progesterone[\/pb_glossary] produced by the [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary] in the ovary stimulates further changes in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] so it is more receptive to implantation of a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary]. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] tissue in the uterine wall.\r\n\r\n \r\nBringing it All Together<\/h1>\r\nIt is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.\r\n\r\n[caption id=\"attachment_5313\" align=\"aligncenter\" width=\"1000\"] Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em>[\/caption]\r\n\r\n \r\n\r\n\r\n18.8 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary] and uterus, and is controlled by [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary] hormones ([pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary]) and ovarian hormones ([pb_glossary id=\"5995\"]estrogen[\/pb_glossary] and [pb_glossary id=\"3411\"]progesterone[\/pb_glossary]).<\/li>\r\n \t
- The female reproductive period is delineated by [pb_glossary id=\"5300\"]menarche[\/pb_glossary], or the first menstrual period, which usually occurs around age 12 or 13; and by [pb_glossary id=\"5255\"]menopause[\/pb_glossary], or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\r\n \t
- The events of the menstrual cycle that take place in the ovaries make up the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It includes the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary] (when a [pb_glossary id=\"5253\"]follicle[\/pb_glossary] and its ovum mature due to rising levels of FSH), [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] (when the [pb_glossary id=\"5099\"]ovum[\/pb_glossary] is released from the ovary due to a rise in estrogen and a surge in LH), and the [pb_glossary id=\"5305\"]luteal phase[\/pb_glossary] (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\r\n \t
- The events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] make up the uterine cycle. It includes [pb_glossary id=\"5310\"]menstruation[\/pb_glossary], which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the [pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary], during which the endometrium builds up again until [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] occurs; and the [pb_glossary id=\"5312\"]secretory phase[\/pb_glossary], which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an [pb_glossary id=\"5262\"]embryo[\/pb_glossary].<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"650\"]<\/li>\r\n \t
- What is the menstrual cycle? Why is the menstrual cycle necessary in order for pregnancy to occur?<\/li>\r\n \t
- What organs are involved in the menstrual cycle?<\/li>\r\n \t
- Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?<\/li>\r\n \t
- Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.<\/li>\r\n \t
- If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.<\/li>\r\n \t
- Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/cjbgZwgdY7Q\r\nWhy do women have periods? TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/5B3Abpv0ysM\r\n
Girl's Rite of Passage | National Geographic, 2007.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 18.8.1<\/strong>\r\n\r\nWaterforPeople_Uganda<\/a> by WaterforPeople_Uganda<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 18.8.2<\/strong>\r\n\r\nOvarian Cycle<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used and adapted under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 18.8.3<\/strong>\r\n\r\nFigure_43_04_04<\/a>\u00a0by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license. (Original: modification of work by Mikael H\u00e4ggstr\u00f6m)\r\n\r\nFigure 18.8.4<\/strong>\r\n\r\nOvarian and menstrual cycle<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0 CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 18.8.5<\/strong>\r\n\r\n1000px-MenstrualCycle2_en.svg<\/a> by Isometrik<\/a> on Wikimedia Common is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span>27.15<\/span>\u00a0<\/span>Hormone levels in ovarian and menstrual cycles<\/span> [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/p>\r\nNational Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=5B3Abpv0ysM&feature=youtu.be<\/p>\r\n
OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In Open Stax, Biology<\/em> (Section 43.4). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction<\/p>\r\nTED-Ed. (2015, October 19). Why do women have periods? YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=cjbgZwgdY7Q&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 18.8.1 It’s healthy to talk about menstruation.<\/em><\/figcaption><\/figure>\n\nTaboo Topic<\/h1>\n<\/div>\n
The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.<\/p>\n
\nWhat Is the Menstrual Cycle?<\/h1>\n<\/div>\n
The\u00a0menstrual cycle<\/a><\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up endometrium<\/a> lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0pregnancy<\/a>\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a follicle<\/a>, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.<\/p>\nMenarche and Menopause<\/h2>\n
The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0menarche<\/a>.<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.<\/p>\nThe cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0menopause<\/a>.<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.<\/p>\nVariation in the Menstrual Cycle<\/h2>\n
The length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.<\/p>\n
\nOvarian Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle that take place in the ovaries make up the\u00a0ovarian cycle<\/a>.<\/strong> It consists of changes that occur in the follicles<\/a> of one of the ovaries<\/a>. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.<\/p>\nFigure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em><\/figcaption><\/figure>\nFollicular Phase<\/h2>\n
The\u00a0follicular phase<\/a><\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles<\/span><\/a><\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle stimulating hormone<\/a> (FSH), which is secreted by the pituitary gland<\/a>. The maturing follicle releases estrogen<\/a>, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span><\/p>\nFigure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em><\/figcaption><\/figure>\nOvulation<\/h2>\n
Ovulation<\/a><\/strong>\u00a0is the second phase of the ovarian cycle<\/a>. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone<\/a> (LH) from the pituitary gland<\/a>. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase<\/a>, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen<\/a> stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte<\/a>.<\/p>\nLuteal Phase<\/h2>\n
The\u00a0luteal phase<\/a><\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH<\/a> and LH<\/a> cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum<\/a>. The corpus luteum secretes progesterone<\/a>, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the endometrium<\/a> in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.<\/p>\n\n- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\n
- If\u00a0fertilization<\/a>\u00a0has occurred so a zygote<\/a> forms and then divides to become a blastocyst<\/a>, the outer layer of the blastocyst produces a\u00a0hormone\u00a0called human chorionic gonadotropin<\/a> (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\n<\/ul>\n\n
Uterine Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle<\/a> that take place in the uterus make up the\u00a0uterine cycle<\/a><\/strong>. This cycle consists of changes that occur mainly in the endometrium<\/a>, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.<\/p>\nFigure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em><\/figcaption><\/figure>\nMenstruation<\/h2>\n
Menstruation<\/a><\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization<\/a> has not taken place during the preceding menstrual cycle. During menstruation, the endometrium<\/a> of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus<\/a>, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.<\/p>\nProliferative Phase<\/h2>\n
The\u00a0proliferative phase<\/a><\/strong>\u00a0is the second phase of the uterine cycle. During this phase, estrogen<\/a> secreted by\u00a0cells\u00a0of the maturing ovarian follicle<\/a> causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix<\/a> of the uterus to secrete larger amounts of thinner mucus that can help\u00a0sperm<\/a>\u00a0swim through the cervix and into the uterus, making fertilization more likely.<\/p>\nSecretory Phase<\/h2>\n
The\u00a0secretory phase<\/a><\/strong>\u00a0is the third and final phase of the uterine cycle<\/a>. During this phase, progesterone<\/a> produced by the corpus luteum<\/a> in the ovary stimulates further changes in the endometrium<\/a> so it is more receptive to implantation of a blastocyst<\/a>. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle<\/a> tissue in the uterine wall.<\/p>\n <\/p>\n
Bringing it All Together<\/h1>\n
It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.<\/p>\nFigure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em><\/figcaption><\/figure>\n <\/p>\n
\n\n\n18.8 Summary<\/span><\/h1>\n<\/header>\n\n\n- The menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries<\/a> and uterus, and is controlled by pituitary gland<\/a> hormones (FSH<\/a> and LH<\/a>) and ovarian hormones (estrogen<\/a> and progesterone<\/a>).<\/li>\n
- The female reproductive period is delineated by menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n
- The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle<\/a>. It includes the follicular phase<\/a> (when a follicle<\/a> and its ovum mature due to rising levels of FSH), ovulation<\/a> (when the ovum<\/a> is released from the ovary due to a rise in estrogen and a surge in LH), and the luteal phase<\/a> (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\n
- The events of the menstrual cycle<\/a> that take place in the uterus<\/a> make up the uterine cycle. It includes menstruation<\/a>, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase<\/a>, during which the endometrium builds up again until ovulation<\/a> occurs; and the secretory phase<\/a>, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n18.8 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
Variation in the Menstrual Cycle<\/h2>\r\nThe length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.\r\n\r\nOvarian Cycle<\/h1>\r\n<\/div>\r\nThe events of the menstrual cycle that take place in the ovaries make up the\u00a0[pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary].<\/strong> It consists of changes that occur in the [pb_glossary id=\"5253\"]follicles[\/pb_glossary] of one of the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary]. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.\r\n\r\n[caption id=\"attachment_5302\" align=\"aligncenter\" width=\"919\"] Figure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em>[\/caption]\r\nFollicular Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5303\"]follicular phase[\/pb_glossary]<\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several [pb_glossary id=\"5253\"]ovarian follicles<\/span>[\/pb_glossary]<\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of [pb_glossary id=\"5206\"]follicle stimulating hormone[\/pb_glossary] (FSH), which is secreted by the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. The maturing follicle releases [pb_glossary id=\"5995\"]estrogen[\/pb_glossary], the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span>\r\n\r\n[caption id=\"attachment_5304\" align=\"aligncenter\" width=\"724\"] Figure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em>[\/caption]\r\nOvulation<\/h2>\r\n[pb_glossary id=\"5239\"]Ovulation[\/pb_glossary]<\/strong>\u00a0is the second phase of the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of [pb_glossary id=\"5205\"]luteinizing hormone[\/pb_glossary] (LH) from the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary], the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary [pb_glossary id=\"5274\"]oocyte[\/pb_glossary].\r\nLuteal Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5305\"]luteal phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, [pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary] cause the Graafian follicle that ovulated the egg to transform into a structure called a [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary]. The corpus luteum secretes [pb_glossary id=\"3411\"]progesterone[\/pb_glossary], which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.\r\n\r\n \t- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\r\n \t
- If\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0has occurred so a [pb_glossary id=\"2471\"]zygote[\/pb_glossary] forms and then divides to become a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary], the outer layer of the blastocyst produces a\u00a0hormone\u00a0called [pb_glossary id=\"5307\"]human chorionic gonadotropin[\/pb_glossary] (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\r\n<\/ul>\r\n\r\n
Uterine Cycle<\/h1>\r\n<\/div>\r\nThe events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the uterus make up the\u00a0[pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]<\/strong>. This cycle consists of changes that occur mainly in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary], which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.\r\n\r\n[caption id=\"attachment_5309\" align=\"aligncenter\" width=\"488\"] Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em>[\/caption]\r\nMenstruation<\/h2>\r\n[pb_glossary id=\"5310\"]Menstruation[\/pb_glossary]<\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] has not taken place during the preceding menstrual cycle. During menstruation, the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] of the uterus, which has built up during the preceding cycle, degenerates and is shed from the [pb_glossary id=\"5102\"]uterus[\/pb_glossary], flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.\r\nProliferative Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary]<\/strong>\u00a0is the second phase of the uterine cycle. During this phase, [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] secreted by\u00a0cells\u00a0of the maturing [pb_glossary id=\"5253\"]ovarian follicle[\/pb_glossary] causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the [pb_glossary id=\"5235\"]cervix[\/pb_glossary] of the uterus to secrete larger amounts of thinner mucus that can help\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]\u00a0swim through the cervix and into the uterus, making fertilization more likely.\r\nSecretory Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5312\"]secretory phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the [pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]. During this phase, [pb_glossary id=\"3411\"]progesterone[\/pb_glossary] produced by the [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary] in the ovary stimulates further changes in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] so it is more receptive to implantation of a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary]. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] tissue in the uterine wall.\r\n\r\n \r\nBringing it All Together<\/h1>\r\nIt is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.\r\n\r\n[caption id=\"attachment_5313\" align=\"aligncenter\" width=\"1000\"] Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em>[\/caption]\r\n\r\n \r\n\r\n\r\n18.8 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary] and uterus, and is controlled by [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary] hormones ([pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary]) and ovarian hormones ([pb_glossary id=\"5995\"]estrogen[\/pb_glossary] and [pb_glossary id=\"3411\"]progesterone[\/pb_glossary]).<\/li>\r\n \t
- The female reproductive period is delineated by [pb_glossary id=\"5300\"]menarche[\/pb_glossary], or the first menstrual period, which usually occurs around age 12 or 13; and by [pb_glossary id=\"5255\"]menopause[\/pb_glossary], or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\r\n \t
- The events of the menstrual cycle that take place in the ovaries make up the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It includes the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary] (when a [pb_glossary id=\"5253\"]follicle[\/pb_glossary] and its ovum mature due to rising levels of FSH), [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] (when the [pb_glossary id=\"5099\"]ovum[\/pb_glossary] is released from the ovary due to a rise in estrogen and a surge in LH), and the [pb_glossary id=\"5305\"]luteal phase[\/pb_glossary] (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\r\n \t
- The events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] make up the uterine cycle. It includes [pb_glossary id=\"5310\"]menstruation[\/pb_glossary], which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the [pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary], during which the endometrium builds up again until [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] occurs; and the [pb_glossary id=\"5312\"]secretory phase[\/pb_glossary], which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an [pb_glossary id=\"5262\"]embryo[\/pb_glossary].<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"650\"]<\/li>\r\n \t
- What is the menstrual cycle? Why is the menstrual cycle necessary in order for pregnancy to occur?<\/li>\r\n \t
- What organs are involved in the menstrual cycle?<\/li>\r\n \t
- Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?<\/li>\r\n \t
- Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.<\/li>\r\n \t
- If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.<\/li>\r\n \t
- Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/cjbgZwgdY7Q\r\nWhy do women have periods? TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/5B3Abpv0ysM\r\n
Girl's Rite of Passage | National Geographic, 2007.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 18.8.1<\/strong>\r\n\r\nWaterforPeople_Uganda<\/a> by WaterforPeople_Uganda<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 18.8.2<\/strong>\r\n\r\nOvarian Cycle<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used and adapted under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 18.8.3<\/strong>\r\n\r\nFigure_43_04_04<\/a>\u00a0by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license. (Original: modification of work by Mikael H\u00e4ggstr\u00f6m)\r\n\r\nFigure 18.8.4<\/strong>\r\n\r\nOvarian and menstrual cycle<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0 CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 18.8.5<\/strong>\r\n\r\n1000px-MenstrualCycle2_en.svg<\/a> by Isometrik<\/a> on Wikimedia Common is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span>27.15<\/span>\u00a0<\/span>Hormone levels in ovarian and menstrual cycles<\/span> [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/p>\r\nNational Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=5B3Abpv0ysM&feature=youtu.be<\/p>\r\n
OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In Open Stax, Biology<\/em> (Section 43.4). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction<\/p>\r\nTED-Ed. (2015, October 19). Why do women have periods? YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=cjbgZwgdY7Q&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 18.8.1 It’s healthy to talk about menstruation.<\/em><\/figcaption><\/figure>\n\nTaboo Topic<\/h1>\n<\/div>\n
The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.<\/p>\n
\nWhat Is the Menstrual Cycle?<\/h1>\n<\/div>\n
The\u00a0menstrual cycle<\/a><\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up endometrium<\/a> lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0pregnancy<\/a>\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a follicle<\/a>, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.<\/p>\nMenarche and Menopause<\/h2>\n
The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0menarche<\/a>.<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.<\/p>\nThe cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0menopause<\/a>.<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.<\/p>\nVariation in the Menstrual Cycle<\/h2>\n
The length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.<\/p>\n
\nOvarian Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle that take place in the ovaries make up the\u00a0ovarian cycle<\/a>.<\/strong> It consists of changes that occur in the follicles<\/a> of one of the ovaries<\/a>. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.<\/p>\nFigure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em><\/figcaption><\/figure>\nFollicular Phase<\/h2>\n
The\u00a0follicular phase<\/a><\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles<\/span><\/a><\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle stimulating hormone<\/a> (FSH), which is secreted by the pituitary gland<\/a>. The maturing follicle releases estrogen<\/a>, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span><\/p>\nFigure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em><\/figcaption><\/figure>\nOvulation<\/h2>\n
Ovulation<\/a><\/strong>\u00a0is the second phase of the ovarian cycle<\/a>. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone<\/a> (LH) from the pituitary gland<\/a>. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase<\/a>, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen<\/a> stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte<\/a>.<\/p>\nLuteal Phase<\/h2>\n
The\u00a0luteal phase<\/a><\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH<\/a> and LH<\/a> cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum<\/a>. The corpus luteum secretes progesterone<\/a>, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the endometrium<\/a> in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.<\/p>\n\n- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\n
- If\u00a0fertilization<\/a>\u00a0has occurred so a zygote<\/a> forms and then divides to become a blastocyst<\/a>, the outer layer of the blastocyst produces a\u00a0hormone\u00a0called human chorionic gonadotropin<\/a> (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\n<\/ul>\n\n
Uterine Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle<\/a> that take place in the uterus make up the\u00a0uterine cycle<\/a><\/strong>. This cycle consists of changes that occur mainly in the endometrium<\/a>, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.<\/p>\nFigure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em><\/figcaption><\/figure>\nMenstruation<\/h2>\n
Menstruation<\/a><\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization<\/a> has not taken place during the preceding menstrual cycle. During menstruation, the endometrium<\/a> of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus<\/a>, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.<\/p>\nProliferative Phase<\/h2>\n
The\u00a0proliferative phase<\/a><\/strong>\u00a0is the second phase of the uterine cycle. During this phase, estrogen<\/a> secreted by\u00a0cells\u00a0of the maturing ovarian follicle<\/a> causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix<\/a> of the uterus to secrete larger amounts of thinner mucus that can help\u00a0sperm<\/a>\u00a0swim through the cervix and into the uterus, making fertilization more likely.<\/p>\nSecretory Phase<\/h2>\n
The\u00a0secretory phase<\/a><\/strong>\u00a0is the third and final phase of the uterine cycle<\/a>. During this phase, progesterone<\/a> produced by the corpus luteum<\/a> in the ovary stimulates further changes in the endometrium<\/a> so it is more receptive to implantation of a blastocyst<\/a>. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle<\/a> tissue in the uterine wall.<\/p>\n <\/p>\n
Bringing it All Together<\/h1>\n
It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.<\/p>\nFigure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em><\/figcaption><\/figure>\n <\/p>\n
\n\n\n18.8 Summary<\/span><\/h1>\n<\/header>\n\n\n- The menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries<\/a> and uterus, and is controlled by pituitary gland<\/a> hormones (FSH<\/a> and LH<\/a>) and ovarian hormones (estrogen<\/a> and progesterone<\/a>).<\/li>\n
- The female reproductive period is delineated by menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n
- The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle<\/a>. It includes the follicular phase<\/a> (when a follicle<\/a> and its ovum mature due to rising levels of FSH), ovulation<\/a> (when the ovum<\/a> is released from the ovary due to a rise in estrogen and a surge in LH), and the luteal phase<\/a> (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\n
- The events of the menstrual cycle<\/a> that take place in the uterus<\/a> make up the uterine cycle. It includes menstruation<\/a>, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase<\/a>, during which the endometrium builds up again until ovulation<\/a> occurs; and the secretory phase<\/a>, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n18.8 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
Ovarian Cycle<\/h1>\r\n<\/div>\r\nThe events of the menstrual cycle that take place in the ovaries make up the\u00a0[pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary].<\/strong> It consists of changes that occur in the [pb_glossary id=\"5253\"]follicles[\/pb_glossary] of one of the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary]. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.\r\n\r\n[caption id=\"attachment_5302\" align=\"aligncenter\" width=\"919\"] Figure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em>[\/caption]\r\nFollicular Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5303\"]follicular phase[\/pb_glossary]<\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several [pb_glossary id=\"5253\"]ovarian follicles<\/span>[\/pb_glossary]<\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of [pb_glossary id=\"5206\"]follicle stimulating hormone[\/pb_glossary] (FSH), which is secreted by the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. The maturing follicle releases [pb_glossary id=\"5995\"]estrogen[\/pb_glossary], the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span>\r\n\r\n[caption id=\"attachment_5304\" align=\"aligncenter\" width=\"724\"] Figure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em>[\/caption]\r\nOvulation<\/h2>\r\n[pb_glossary id=\"5239\"]Ovulation[\/pb_glossary]<\/strong>\u00a0is the second phase of the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of [pb_glossary id=\"5205\"]luteinizing hormone[\/pb_glossary] (LH) from the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary], the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary [pb_glossary id=\"5274\"]oocyte[\/pb_glossary].\r\nLuteal Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5305\"]luteal phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, [pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary] cause the Graafian follicle that ovulated the egg to transform into a structure called a [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary]. The corpus luteum secretes [pb_glossary id=\"3411\"]progesterone[\/pb_glossary], which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.\r\n\r\n \t- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\r\n \t
- If\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0has occurred so a [pb_glossary id=\"2471\"]zygote[\/pb_glossary] forms and then divides to become a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary], the outer layer of the blastocyst produces a\u00a0hormone\u00a0called [pb_glossary id=\"5307\"]human chorionic gonadotropin[\/pb_glossary] (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\r\n<\/ul>\r\n\r\n
Uterine Cycle<\/h1>\r\n<\/div>\r\nThe events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the uterus make up the\u00a0[pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]<\/strong>. This cycle consists of changes that occur mainly in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary], which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.\r\n\r\n[caption id=\"attachment_5309\" align=\"aligncenter\" width=\"488\"] Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em>[\/caption]\r\nMenstruation<\/h2>\r\n[pb_glossary id=\"5310\"]Menstruation[\/pb_glossary]<\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] has not taken place during the preceding menstrual cycle. During menstruation, the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] of the uterus, which has built up during the preceding cycle, degenerates and is shed from the [pb_glossary id=\"5102\"]uterus[\/pb_glossary], flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.\r\nProliferative Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary]<\/strong>\u00a0is the second phase of the uterine cycle. During this phase, [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] secreted by\u00a0cells\u00a0of the maturing [pb_glossary id=\"5253\"]ovarian follicle[\/pb_glossary] causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the [pb_glossary id=\"5235\"]cervix[\/pb_glossary] of the uterus to secrete larger amounts of thinner mucus that can help\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]\u00a0swim through the cervix and into the uterus, making fertilization more likely.\r\nSecretory Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5312\"]secretory phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the [pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]. During this phase, [pb_glossary id=\"3411\"]progesterone[\/pb_glossary] produced by the [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary] in the ovary stimulates further changes in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] so it is more receptive to implantation of a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary]. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] tissue in the uterine wall.\r\n\r\n \r\nBringing it All Together<\/h1>\r\nIt is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.\r\n\r\n[caption id=\"attachment_5313\" align=\"aligncenter\" width=\"1000\"] Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em>[\/caption]\r\n\r\n \r\n\r\n\r\n18.8 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary] and uterus, and is controlled by [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary] hormones ([pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary]) and ovarian hormones ([pb_glossary id=\"5995\"]estrogen[\/pb_glossary] and [pb_glossary id=\"3411\"]progesterone[\/pb_glossary]).<\/li>\r\n \t
- The female reproductive period is delineated by [pb_glossary id=\"5300\"]menarche[\/pb_glossary], or the first menstrual period, which usually occurs around age 12 or 13; and by [pb_glossary id=\"5255\"]menopause[\/pb_glossary], or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\r\n \t
- The events of the menstrual cycle that take place in the ovaries make up the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It includes the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary] (when a [pb_glossary id=\"5253\"]follicle[\/pb_glossary] and its ovum mature due to rising levels of FSH), [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] (when the [pb_glossary id=\"5099\"]ovum[\/pb_glossary] is released from the ovary due to a rise in estrogen and a surge in LH), and the [pb_glossary id=\"5305\"]luteal phase[\/pb_glossary] (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\r\n \t
- The events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] make up the uterine cycle. It includes [pb_glossary id=\"5310\"]menstruation[\/pb_glossary], which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the [pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary], during which the endometrium builds up again until [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] occurs; and the [pb_glossary id=\"5312\"]secretory phase[\/pb_glossary], which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an [pb_glossary id=\"5262\"]embryo[\/pb_glossary].<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"650\"]<\/li>\r\n \t
- What is the menstrual cycle? Why is the menstrual cycle necessary in order for pregnancy to occur?<\/li>\r\n \t
- What organs are involved in the menstrual cycle?<\/li>\r\n \t
- Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?<\/li>\r\n \t
- Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.<\/li>\r\n \t
- If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.<\/li>\r\n \t
- Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/cjbgZwgdY7Q\r\nWhy do women have periods? TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/5B3Abpv0ysM\r\n
Girl's Rite of Passage | National Geographic, 2007.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 18.8.1<\/strong>\r\n\r\nWaterforPeople_Uganda<\/a> by WaterforPeople_Uganda<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 18.8.2<\/strong>\r\n\r\nOvarian Cycle<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used and adapted under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 18.8.3<\/strong>\r\n\r\nFigure_43_04_04<\/a>\u00a0by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license. (Original: modification of work by Mikael H\u00e4ggstr\u00f6m)\r\n\r\nFigure 18.8.4<\/strong>\r\n\r\nOvarian and menstrual cycle<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0 CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 18.8.5<\/strong>\r\n\r\n1000px-MenstrualCycle2_en.svg<\/a> by Isometrik<\/a> on Wikimedia Common is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span>27.15<\/span>\u00a0<\/span>Hormone levels in ovarian and menstrual cycles<\/span> [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/p>\r\nNational Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=5B3Abpv0ysM&feature=youtu.be<\/p>\r\n
OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In Open Stax, Biology<\/em> (Section 43.4). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction<\/p>\r\nTED-Ed. (2015, October 19). Why do women have periods? YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=cjbgZwgdY7Q&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 18.8.1 It’s healthy to talk about menstruation.<\/em><\/figcaption><\/figure>\n\nTaboo Topic<\/h1>\n<\/div>\n
The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.<\/p>\n
\nWhat Is the Menstrual Cycle?<\/h1>\n<\/div>\n
The\u00a0menstrual cycle<\/a><\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up endometrium<\/a> lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0pregnancy<\/a>\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a follicle<\/a>, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.<\/p>\nMenarche and Menopause<\/h2>\n
The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0menarche<\/a>.<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.<\/p>\nThe cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0menopause<\/a>.<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.<\/p>\nVariation in the Menstrual Cycle<\/h2>\n
The length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.<\/p>\n
\nOvarian Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle that take place in the ovaries make up the\u00a0ovarian cycle<\/a>.<\/strong> It consists of changes that occur in the follicles<\/a> of one of the ovaries<\/a>. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.<\/p>\nFigure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em><\/figcaption><\/figure>\nFollicular Phase<\/h2>\n
The\u00a0follicular phase<\/a><\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles<\/span><\/a><\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle stimulating hormone<\/a> (FSH), which is secreted by the pituitary gland<\/a>. The maturing follicle releases estrogen<\/a>, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span><\/p>\nFigure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em><\/figcaption><\/figure>\nOvulation<\/h2>\n
Ovulation<\/a><\/strong>\u00a0is the second phase of the ovarian cycle<\/a>. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone<\/a> (LH) from the pituitary gland<\/a>. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase<\/a>, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen<\/a> stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte<\/a>.<\/p>\nLuteal Phase<\/h2>\n
The\u00a0luteal phase<\/a><\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH<\/a> and LH<\/a> cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum<\/a>. The corpus luteum secretes progesterone<\/a>, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the endometrium<\/a> in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.<\/p>\n\n- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\n
- If\u00a0fertilization<\/a>\u00a0has occurred so a zygote<\/a> forms and then divides to become a blastocyst<\/a>, the outer layer of the blastocyst produces a\u00a0hormone\u00a0called human chorionic gonadotropin<\/a> (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\n<\/ul>\n\n
Uterine Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle<\/a> that take place in the uterus make up the\u00a0uterine cycle<\/a><\/strong>. This cycle consists of changes that occur mainly in the endometrium<\/a>, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.<\/p>\nFigure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em><\/figcaption><\/figure>\nMenstruation<\/h2>\n
Menstruation<\/a><\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization<\/a> has not taken place during the preceding menstrual cycle. During menstruation, the endometrium<\/a> of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus<\/a>, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.<\/p>\nProliferative Phase<\/h2>\n
The\u00a0proliferative phase<\/a><\/strong>\u00a0is the second phase of the uterine cycle. During this phase, estrogen<\/a> secreted by\u00a0cells\u00a0of the maturing ovarian follicle<\/a> causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix<\/a> of the uterus to secrete larger amounts of thinner mucus that can help\u00a0sperm<\/a>\u00a0swim through the cervix and into the uterus, making fertilization more likely.<\/p>\nSecretory Phase<\/h2>\n
The\u00a0secretory phase<\/a><\/strong>\u00a0is the third and final phase of the uterine cycle<\/a>. During this phase, progesterone<\/a> produced by the corpus luteum<\/a> in the ovary stimulates further changes in the endometrium<\/a> so it is more receptive to implantation of a blastocyst<\/a>. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle<\/a> tissue in the uterine wall.<\/p>\n <\/p>\n
Bringing it All Together<\/h1>\n
It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.<\/p>\nFigure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em><\/figcaption><\/figure>\n <\/p>\n
\n\n\n18.8 Summary<\/span><\/h1>\n<\/header>\n\n\n- The menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries<\/a> and uterus, and is controlled by pituitary gland<\/a> hormones (FSH<\/a> and LH<\/a>) and ovarian hormones (estrogen<\/a> and progesterone<\/a>).<\/li>\n
- The female reproductive period is delineated by menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n
- The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle<\/a>. It includes the follicular phase<\/a> (when a follicle<\/a> and its ovum mature due to rising levels of FSH), ovulation<\/a> (when the ovum<\/a> is released from the ovary due to a rise in estrogen and a surge in LH), and the luteal phase<\/a> (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\n
- The events of the menstrual cycle<\/a> that take place in the uterus<\/a> make up the uterine cycle. It includes menstruation<\/a>, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase<\/a>, during which the endometrium builds up again until ovulation<\/a> occurs; and the secretory phase<\/a>, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n18.8 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
Figure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em>[\/caption]\r\nOvulation<\/h2>\r\n[pb_glossary id=\"5239\"]Ovulation[\/pb_glossary]<\/strong>\u00a0is the second phase of the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of [pb_glossary id=\"5205\"]luteinizing hormone[\/pb_glossary] (LH) from the [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary]. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary], the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary [pb_glossary id=\"5274\"]oocyte[\/pb_glossary].\r\nLuteal Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5305\"]luteal phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, [pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary] cause the Graafian follicle that ovulated the egg to transform into a structure called a [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary]. The corpus luteum secretes [pb_glossary id=\"3411\"]progesterone[\/pb_glossary], which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.\r\n\r\n \t- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\r\n \t
- If\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0has occurred so a [pb_glossary id=\"2471\"]zygote[\/pb_glossary] forms and then divides to become a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary], the outer layer of the blastocyst produces a\u00a0hormone\u00a0called [pb_glossary id=\"5307\"]human chorionic gonadotropin[\/pb_glossary] (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\r\n<\/ul>\r\n\r\n
Uterine Cycle<\/h1>\r\n<\/div>\r\nThe events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the uterus make up the\u00a0[pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]<\/strong>. This cycle consists of changes that occur mainly in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary], which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.\r\n\r\n[caption id=\"attachment_5309\" align=\"aligncenter\" width=\"488\"] Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em>[\/caption]\r\nMenstruation<\/h2>\r\n[pb_glossary id=\"5310\"]Menstruation[\/pb_glossary]<\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] has not taken place during the preceding menstrual cycle. During menstruation, the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] of the uterus, which has built up during the preceding cycle, degenerates and is shed from the [pb_glossary id=\"5102\"]uterus[\/pb_glossary], flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.\r\nProliferative Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary]<\/strong>\u00a0is the second phase of the uterine cycle. During this phase, [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] secreted by\u00a0cells\u00a0of the maturing [pb_glossary id=\"5253\"]ovarian follicle[\/pb_glossary] causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the [pb_glossary id=\"5235\"]cervix[\/pb_glossary] of the uterus to secrete larger amounts of thinner mucus that can help\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]\u00a0swim through the cervix and into the uterus, making fertilization more likely.\r\nSecretory Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5312\"]secretory phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the [pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]. During this phase, [pb_glossary id=\"3411\"]progesterone[\/pb_glossary] produced by the [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary] in the ovary stimulates further changes in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] so it is more receptive to implantation of a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary]. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] tissue in the uterine wall.\r\n\r\n \r\nBringing it All Together<\/h1>\r\nIt is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.\r\n\r\n[caption id=\"attachment_5313\" align=\"aligncenter\" width=\"1000\"] Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em>[\/caption]\r\n\r\n \r\n\r\n\r\n18.8 Summary<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- The [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary] and uterus, and is controlled by [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary] hormones ([pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary]) and ovarian hormones ([pb_glossary id=\"5995\"]estrogen[\/pb_glossary] and [pb_glossary id=\"3411\"]progesterone[\/pb_glossary]).<\/li>\r\n \t
- The female reproductive period is delineated by [pb_glossary id=\"5300\"]menarche[\/pb_glossary], or the first menstrual period, which usually occurs around age 12 or 13; and by [pb_glossary id=\"5255\"]menopause[\/pb_glossary], or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\r\n \t
- The events of the menstrual cycle that take place in the ovaries make up the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It includes the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary] (when a [pb_glossary id=\"5253\"]follicle[\/pb_glossary] and its ovum mature due to rising levels of FSH), [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] (when the [pb_glossary id=\"5099\"]ovum[\/pb_glossary] is released from the ovary due to a rise in estrogen and a surge in LH), and the [pb_glossary id=\"5305\"]luteal phase[\/pb_glossary] (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\r\n \t
- The events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] make up the uterine cycle. It includes [pb_glossary id=\"5310\"]menstruation[\/pb_glossary], which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the [pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary], during which the endometrium builds up again until [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] occurs; and the [pb_glossary id=\"5312\"]secretory phase[\/pb_glossary], which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an [pb_glossary id=\"5262\"]embryo[\/pb_glossary].<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Review Questions<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\n \t- [h5p id=\"650\"]<\/li>\r\n \t
- What is the menstrual cycle? Why is the menstrual cycle necessary in order for pregnancy to occur?<\/li>\r\n \t
- What organs are involved in the menstrual cycle?<\/li>\r\n \t
- Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?<\/li>\r\n \t
- Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.<\/li>\r\n \t
- If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.<\/li>\r\n \t
- Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n18.8 Explore More<\/span><\/h1>\r\n<\/header>\r\n\r\n\r\nhttps:\/\/youtu.be\/cjbgZwgdY7Q\r\nWhy do women have periods? TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/5B3Abpv0ysM\r\n
Girl's Rite of Passage | National Geographic, 2007.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 18.8.1<\/strong>\r\n\r\nWaterforPeople_Uganda<\/a> by WaterforPeople_Uganda<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 18.8.2<\/strong>\r\n\r\nOvarian Cycle<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used and adapted under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 18.8.3<\/strong>\r\n\r\nFigure_43_04_04<\/a>\u00a0by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license. (Original: modification of work by Mikael H\u00e4ggstr\u00f6m)\r\n\r\nFigure 18.8.4<\/strong>\r\n\r\nOvarian and menstrual cycle<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0 CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 18.8.5<\/strong>\r\n\r\n1000px-MenstrualCycle2_en.svg<\/a> by Isometrik<\/a> on Wikimedia Common is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\nReferences<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span>27.15<\/span>\u00a0<\/span>Hormone levels in ovarian and menstrual cycles<\/span> [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/p>\r\nNational Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=5B3Abpv0ysM&feature=youtu.be<\/p>\r\n
OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In Open Stax, Biology<\/em> (Section 43.4). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction<\/p>\r\nTED-Ed. (2015, October 19). Why do women have periods? YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=cjbgZwgdY7Q&feature=youtu.be<\/p>","rendered":"
<\/p>\nFigure 18.8.1 It’s healthy to talk about menstruation.<\/em><\/figcaption><\/figure>\n\nTaboo Topic<\/h1>\n<\/div>\n
The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.<\/p>\n
\nWhat Is the Menstrual Cycle?<\/h1>\n<\/div>\n
The\u00a0menstrual cycle<\/a><\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up endometrium<\/a> lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0pregnancy<\/a>\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a follicle<\/a>, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.<\/p>\nMenarche and Menopause<\/h2>\n
The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0menarche<\/a>.<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.<\/p>\nThe cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0menopause<\/a>.<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.<\/p>\nVariation in the Menstrual Cycle<\/h2>\n
The length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.<\/p>\n
\nOvarian Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle that take place in the ovaries make up the\u00a0ovarian cycle<\/a>.<\/strong> It consists of changes that occur in the follicles<\/a> of one of the ovaries<\/a>. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.<\/p>\nFigure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em><\/figcaption><\/figure>\nFollicular Phase<\/h2>\n
The\u00a0follicular phase<\/a><\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles<\/span><\/a><\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle stimulating hormone<\/a> (FSH), which is secreted by the pituitary gland<\/a>. The maturing follicle releases estrogen<\/a>, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span><\/p>\nFigure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em><\/figcaption><\/figure>\nOvulation<\/h2>\n
Ovulation<\/a><\/strong>\u00a0is the second phase of the ovarian cycle<\/a>. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone<\/a> (LH) from the pituitary gland<\/a>. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase<\/a>, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen<\/a> stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte<\/a>.<\/p>\nLuteal Phase<\/h2>\n
The\u00a0luteal phase<\/a><\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH<\/a> and LH<\/a> cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum<\/a>. The corpus luteum secretes progesterone<\/a>, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the endometrium<\/a> in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.<\/p>\n\n- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\n
- If\u00a0fertilization<\/a>\u00a0has occurred so a zygote<\/a> forms and then divides to become a blastocyst<\/a>, the outer layer of the blastocyst produces a\u00a0hormone\u00a0called human chorionic gonadotropin<\/a> (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\n<\/ul>\n\n
Uterine Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle<\/a> that take place in the uterus make up the\u00a0uterine cycle<\/a><\/strong>. This cycle consists of changes that occur mainly in the endometrium<\/a>, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.<\/p>\nFigure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em><\/figcaption><\/figure>\nMenstruation<\/h2>\n
Menstruation<\/a><\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization<\/a> has not taken place during the preceding menstrual cycle. During menstruation, the endometrium<\/a> of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus<\/a>, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.<\/p>\nProliferative Phase<\/h2>\n
The\u00a0proliferative phase<\/a><\/strong>\u00a0is the second phase of the uterine cycle. During this phase, estrogen<\/a> secreted by\u00a0cells\u00a0of the maturing ovarian follicle<\/a> causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix<\/a> of the uterus to secrete larger amounts of thinner mucus that can help\u00a0sperm<\/a>\u00a0swim through the cervix and into the uterus, making fertilization more likely.<\/p>\nSecretory Phase<\/h2>\n
The\u00a0secretory phase<\/a><\/strong>\u00a0is the third and final phase of the uterine cycle<\/a>. During this phase, progesterone<\/a> produced by the corpus luteum<\/a> in the ovary stimulates further changes in the endometrium<\/a> so it is more receptive to implantation of a blastocyst<\/a>. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle<\/a> tissue in the uterine wall.<\/p>\n <\/p>\n
Bringing it All Together<\/h1>\n
It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.<\/p>\nFigure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em><\/figcaption><\/figure>\n <\/p>\n
\n\n\n18.8 Summary<\/span><\/h1>\n<\/header>\n\n\n- The menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries<\/a> and uterus, and is controlled by pituitary gland<\/a> hormones (FSH<\/a> and LH<\/a>) and ovarian hormones (estrogen<\/a> and progesterone<\/a>).<\/li>\n
- The female reproductive period is delineated by menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n
- The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle<\/a>. It includes the follicular phase<\/a> (when a follicle<\/a> and its ovum mature due to rising levels of FSH), ovulation<\/a> (when the ovum<\/a> is released from the ovary due to a rise in estrogen and a surge in LH), and the luteal phase<\/a> (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\n
- The events of the menstrual cycle<\/a> that take place in the uterus<\/a> make up the uterine cycle. It includes menstruation<\/a>, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase<\/a>, during which the endometrium builds up again until ovulation<\/a> occurs; and the secretory phase<\/a>, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n\n
\n18.8 Review Questions<\/span><\/h1>\n<\/header>\n\n\n- \n\n
- \r\n \t
- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\r\n \t
- If\u00a0[pb_glossary id=\"6009\"]fertilization[\/pb_glossary]\u00a0has occurred so a [pb_glossary id=\"2471\"]zygote[\/pb_glossary] forms and then divides to become a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary], the outer layer of the blastocyst produces a\u00a0hormone\u00a0called [pb_glossary id=\"5307\"]human chorionic gonadotropin[\/pb_glossary] (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\r\n<\/ul>\r\n\r\n
Uterine Cycle<\/h1>\r\n<\/div>\r\nThe events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the uterus make up the\u00a0[pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]<\/strong>. This cycle consists of changes that occur mainly in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary], which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.\r\n\r\n[caption id=\"attachment_5309\" align=\"aligncenter\" width=\"488\"]
Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em>[\/caption]\r\nMenstruation<\/h2>\r\n[pb_glossary id=\"5310\"]Menstruation[\/pb_glossary]<\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if [pb_glossary id=\"6009\"]fertilization[\/pb_glossary] has not taken place during the preceding menstrual cycle. During menstruation, the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] of the uterus, which has built up during the preceding cycle, degenerates and is shed from the [pb_glossary id=\"5102\"]uterus[\/pb_glossary], flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.\r\n
Proliferative Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary]<\/strong>\u00a0is the second phase of the uterine cycle. During this phase, [pb_glossary id=\"5995\"]estrogen[\/pb_glossary] secreted by\u00a0cells\u00a0of the maturing [pb_glossary id=\"5253\"]ovarian follicle[\/pb_glossary] causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the [pb_glossary id=\"5235\"]cervix[\/pb_glossary] of the uterus to secrete larger amounts of thinner mucus that can help\u00a0[pb_glossary id=\"2506\"]sperm[\/pb_glossary]\u00a0swim through the cervix and into the uterus, making fertilization more likely.\r\n
Secretory Phase<\/h2>\r\nThe\u00a0[pb_glossary id=\"5312\"]secretory phase[\/pb_glossary]<\/strong>\u00a0is the third and final phase of the [pb_glossary id=\"5308\"]uterine cycle[\/pb_glossary]. During this phase, [pb_glossary id=\"3411\"]progesterone[\/pb_glossary] produced by the [pb_glossary id=\"5277\"]corpus luteum[\/pb_glossary] in the ovary stimulates further changes in the [pb_glossary id=\"5241\"]endometrium[\/pb_glossary] so it is more receptive to implantation of a [pb_glossary id=\"5279\"]blastocyst[\/pb_glossary]. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of [pb_glossary id=\"2982\"]smooth muscle[\/pb_glossary] tissue in the uterine wall.\r\n\r\n \r\n
Bringing it All Together<\/h1>\r\nIt is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.\r\n\r\n[caption id=\"attachment_5313\" align=\"aligncenter\" width=\"1000\"]
Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em>[\/caption]\r\n\r\n \r\n\r\n\r\n 18.8 Summary<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- The [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] for [pb_glossary id=\"5280\"]pregnancy[\/pb_glossary]. It involves changes in both the [pb_glossary id=\"3487\"]ovaries[\/pb_glossary] and uterus, and is controlled by [pb_glossary id=\"2938\"]pituitary gland[\/pb_glossary] hormones ([pb_glossary id=\"5206\"]FSH[\/pb_glossary] and [pb_glossary id=\"5306\"]LH[\/pb_glossary]) and ovarian hormones ([pb_glossary id=\"5995\"]estrogen[\/pb_glossary] and [pb_glossary id=\"3411\"]progesterone[\/pb_glossary]).<\/li>\r\n \t
- The female reproductive period is delineated by [pb_glossary id=\"5300\"]menarche[\/pb_glossary], or the first menstrual period, which usually occurs around age 12 or 13; and by [pb_glossary id=\"5255\"]menopause[\/pb_glossary], or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\r\n \t
- The events of the menstrual cycle that take place in the ovaries make up the [pb_glossary id=\"5301\"]ovarian cycle[\/pb_glossary]. It includes the [pb_glossary id=\"5303\"]follicular phase[\/pb_glossary] (when a [pb_glossary id=\"5253\"]follicle[\/pb_glossary] and its ovum mature due to rising levels of FSH), [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] (when the [pb_glossary id=\"5099\"]ovum[\/pb_glossary] is released from the ovary due to a rise in estrogen and a surge in LH), and the [pb_glossary id=\"5305\"]luteal phase[\/pb_glossary] (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\r\n \t
- The events of the [pb_glossary id=\"5293\"]menstrual cycle[\/pb_glossary] that take place in the [pb_glossary id=\"5102\"]uterus[\/pb_glossary] make up the uterine cycle. It includes [pb_glossary id=\"5310\"]menstruation[\/pb_glossary], which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the [pb_glossary id=\"5311\"]proliferative phase[\/pb_glossary], during which the endometrium builds up again until [pb_glossary id=\"5239\"]ovulation[\/pb_glossary] occurs; and the [pb_glossary id=\"5312\"]secretory phase[\/pb_glossary], which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an [pb_glossary id=\"5262\"]embryo[\/pb_glossary].<\/li>\r\n<\/ul>\r\n<\/div>\r\n<\/div>\r\n
\r\n 18.8 Review Questions<\/span><\/h1>\r\n<\/header>\r\n
\r\n- \r\n \t
- [h5p id=\"650\"]<\/li>\r\n \t
- What is the menstrual cycle? Why is the menstrual cycle necessary in order for pregnancy to occur?<\/li>\r\n \t
- What organs are involved in the menstrual cycle?<\/li>\r\n \t
- Identify the two major events that mark the beginning and end of the reproductive period in females. When do these events typically occur?<\/li>\r\n \t
- Discuss the average length of the menstrual cycle and menstruation, as well as variations that are considered normal.<\/li>\r\n \t
- If the LH surge did not occur in a menstrual cycle, what do you think would happen? Explain your answer.<\/li>\r\n \t
- Give one reason why FSH and LH levels drop in the luteal phase of the menstrual cycle.<\/li>\r\n<\/ol>\r\n<\/div>\r\n<\/div>\r\n
\r\n 18.8 Explore More<\/span><\/h1>\r\n<\/header>\r\n
\r\n\r\nhttps:\/\/youtu.be\/cjbgZwgdY7Q\r\nWhy do women have periods? TED-Ed, 2015.<\/p>\r\nhttps:\/\/youtu.be\/5B3Abpv0ysM\r\n
Girl's Rite of Passage | National Geographic, 2007.<\/p>\r\n\r\n<\/div>\r\n<\/div>\r\n \r\n\r\n<\/div>\r\n
Attributions<\/h2>\r\nFigure 18.8.1<\/strong>\r\n\r\nWaterforPeople_Uganda<\/a> by WaterforPeople_Uganda<\/a> on Wikimedia Commons is used under a CC BY 2.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/2.0) license.\r\n\r\nFigure 18.8.2<\/strong>\r\n\r\nOvarian Cycle<\/a> by CNX OpenStax<\/a> on Wikimedia Commons is used and adapted under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license.\r\n\r\nFigure 18.8.3<\/strong>\r\n\r\nFigure_43_04_04<\/a>\u00a0by CNX OpenStax<\/a> on Wikimedia Commons is used under a CC BY 4.0 <\/a>(https:\/\/creativecommons.org\/licenses\/by\/4.0) license. (Original: modification of work by Mikael H\u00e4ggstr\u00f6m)\r\n\r\nFigure 18.8.4<\/strong>\r\n\r\nOvarian and menstrual cycle<\/a>\u00a0by OpenStax College<\/a> on Wikimedia Commons is used under a\u00a0 CC BY 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by\/3.0) license.\r\n\r\nFigure 18.8.5<\/strong>\r\n\r\n1000px-MenstrualCycle2_en.svg<\/a> by Isometrik<\/a> on Wikimedia Common is used under a CC BY-SA 3.0<\/a> (https:\/\/creativecommons.org\/licenses\/by-sa\/3.0) license.\r\n
References<\/h2>\r\n
Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2013, June 19). Figure\u00a0<\/span>27.15<\/span>\u00a0<\/span>Hormone levels in ovarian and menstrual cycles<\/span> [digital image].\u00a0 In Anatomy and Physiology<\/em> (Section 27.2). OpenStax. https:\/\/openstax.org\/books\/anatomy-and-physiology\/pages\/27-2-anatomy-and-physiology-of-the-female-reproductive-system<\/p>\r\n
National Geographic. (2007, May 31). Girl's rite of passage | National Geographic. YouTube. https:\/\/www.youtube.com\/watch?v=5B3Abpv0ysM&feature=youtu.be<\/p>\r\n
OpenStax. (2016, May 27) Figure 4 Rising and falling hormone levels result in progression of the ovarian and menstrual cycles [digital image]. In Open Stax, Biology<\/em> (Section 43.4). OpenStax CNX. https:\/\/cnx.org\/contents\/GFy_h8cu@10.53:Ha3dnFEx@6\/Hormonal-Control-of-Human-Reproduction<\/p>\r\n
TED-Ed. (2015, October 19). Why do women have periods? YouTube.\u00a0 https:\/\/www.youtube.com\/watch?v=cjbgZwgdY7Q&feature=youtu.be<\/p>","rendered":"
<\/p>\n
Figure 18.8.1 It’s healthy to talk about menstruation.<\/em><\/figcaption><\/figure>\n \nTaboo Topic<\/h1>\n<\/div>\n
The banner in Figure 18.8.1 was carried in a 2014 march in Uganda as part of the celebration of Menstrual Hygiene Day. Menstrual Hygiene Day is an awareness day on May 28 of each year that aims to raise awareness worldwide about menstruation and menstrual hygiene. Maintaining good menstrual hygiene is difficult in developing countries like Uganda because of taboos on discussing menstruation and lack of availability of menstrual hygiene products. Poor menstrual hygiene, in turn, can lead to embarrassment, degradation, and reproductive health problems in females. May 28 was chosen as Menstrual Hygiene Day because of its symbolism. May is the fifth month of the year, and most women average five days of menstrual bleeding each month. The 28th day was chosen because the menstrual cycle averages about 28 days.<\/p>\n
\nWhat Is the Menstrual Cycle?<\/h1>\n<\/div>\n
The\u00a0menstrual cycle<\/a><\/strong> refers to natural changes that occur in the female reproductive system each month during the reproductive years. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries and the uterus, and is controlled by pituitary and ovarian hormones. Day 1 of the cycle is the first day of the menstrual period,\u00a0when bleeding from the uterus begins as the built-up endometrium<\/a> lining the uterus is shed. The endometrium builds up again during the remainder of the cycle, only to be shed again during the beginning of the next cycle if\u00a0pregnancy<\/a>\u00a0does not occur. In the ovaries, the menstrual cycle includes the\u00a0development\u00a0of a follicle<\/a>, ovulation of a secondary oocyte, and then degeneration of the follicle if pregnancy does not occur. Both uterine and ovarian changes during the menstrual cycle are generally divided into three phases, although the phases are not the same in the two organs.<\/p>\n
Menarche and Menopause<\/h2>\n
The female reproductive years are delineated by the start and stop of the menstrual cycle. The first menstrual period usually occurs around 12 or 13 years of age, an event that is known as\u00a0menarche<\/a>.<\/strong>\u00a0There is considerable variation among individuals in the age at menarche. It may occasionally occur as early as eight years of age or as late as 16 years of age and still be considered normal. The average age is generally later in the developing world, and earlier in the developed world. This variation is thought to be largely attributable to nutritional differences.<\/p>\n
The cessation of menstrual cycles at the end of a woman\u2019s reproductive years is termed\u00a0menopause<\/a>.<\/strong>\u00a0The average age of menopause is 52 years, but it may occur normally at any age between about 45 and 55 years of age. The age of menopause varies due to a variety of biological and environmental factors.\u00a0It\u00a0may occur earlier as a result of certain illnesses or medical treatments.<\/p>\n
Variation in the Menstrual Cycle<\/h2>\n
The length of the menstrual cycle \u2014 as well as its phases \u2014 may vary considerably, not only among different women, but also from month to month for a given woman. The average length of time between the first day of one menstrual period and the first day of the next menstrual period is 28 days, but it may range from 21 days to 45 days. Cycles are considered regular when a woman\u2019s longest and shortest cycles differ by less than eight days. The menstrual period itself is usually about five days long, but it may vary in length from about two days to seven days.<\/p>\n
\nOvarian Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle that take place in the ovaries make up the\u00a0ovarian cycle<\/a>.<\/strong> It consists of changes that occur in the follicles<\/a> of one of the ovaries<\/a>. The ovarian cycle is divided into the following three phases: follicular phase, ovulation, and luteal phase. These phases are illustrated in Figure 18.8.2.<\/p>\n
Figure 18.8.2 The phases and days of the ovarian cycle are shown in this diagram. The ovarian cycle depicted in the diagram represents a cycle in which fertilization does not occur so the corpus luteum degenerates during the luteal phase.<\/em><\/figcaption><\/figure>\n Follicular Phase<\/h2>\n
The\u00a0follicular phase<\/a><\/strong>\u00a0is the first phase of the ovarian cycle. It generally lasts about 12 to 14 days for a 28-day menstrual cycle. During this phase, several ovarian follicles<\/span><\/a><\/span> are stimulated to begin maturing, but usually only one \u2014 called the Graafian follicle \u2014 matures completely so it is ready to release an egg. The other maturing follicles stop growing and disintegrate. Follicular development occurs because of a rise in the blood level of follicle stimulating hormone<\/a> (FSH), which is secreted by the pituitary gland<\/a>. The maturing follicle releases estrogen<\/a>, the level of which rises throughout the follicular phase. You can see these and other changes in hormone levels that occur during the menstrual cycle in the following chart.<\/span><\/p>\n
Figure 18.8.3 FSH and estrogen increase during the first half of the menstrual cycle. LH surges shortly before ovulation occurs due to the rise in estrogen.<\/em><\/figcaption><\/figure>\n Ovulation<\/h2>\n
Ovulation<\/a><\/strong>\u00a0is the second phase of the ovarian cycle<\/a>. It usually occurs around day 14 of a 28-day menstrual cycle. During this phase, the Graafian follicle ruptures and releases its ovum. Ovulation is stimulated by a sudden rise in the blood level of luteinizing hormone<\/a> (LH) from the pituitary gland<\/a>. This is called the LH surge. You can see the LH surge in the top hormone graph in Figure 18.8.3. The LH surge generally starts around day 12 of the cycle and lasts for a day or two. The surge in LH is triggered by a continued rise in estrogen from the maturing follicle in the ovary. During the follicular phase<\/a>, the rising estrogen level actually suppresses LH secretion by the pituitary gland. However, by the time the follicular phase is nearing its end, the level of estrogen reaches a threshold level above which this effect is reversed, and estrogen<\/a> stimulates the release of a large amount of LH. The surge in LH matures the ovum and weakens the wall of the follicle, causing the fully developed follicle to release its secondary oocyte<\/a>.<\/p>\n
Luteal Phase<\/h2>\n
The\u00a0luteal phase<\/a><\/strong>\u00a0is the third and final phase of the ovarian cycle. It typically lasts about 14 days in a 28-day menstrual cycle. At the beginning of the luteal phase, FSH<\/a> and LH<\/a> cause the Graafian follicle that ovulated the egg to transform into a structure called a corpus luteum<\/a>. The corpus luteum secretes progesterone<\/a>, which in turn suppresses FSH and LH production by the pituitary gland and stimulates the continued buildup of the endometrium<\/a> in the uterus. How this phase ends depends on whether or not the ovum has been fertilized.<\/p>\n
- \n
- If\u00a0fertilization\u00a0has not occurred, the falling levels of FSH and LH during the luteal phase cause the corpus luteum to atrophy, so its production of progesterone declines. Without a high level of progesterone to maintain it, the endometrium starts to break down. By the end of the luteal phase, the endometrium can no longer be maintained, and the next menstrual cycle begins with the shedding of the endometrium (menses).<\/li>\n
- If\u00a0fertilization<\/a>\u00a0has occurred so a zygote<\/a> forms and then divides to become a blastocyst<\/a>, the outer layer of the blastocyst produces a\u00a0hormone\u00a0called human chorionic gonadotropin<\/a> (HCG). This hormone is very similar to LH and preserves the corpus luteum. The corpus luteum can then continue to secrete progesterone to maintain the new pregnancy.<\/li>\n<\/ul>\n\n
Uterine Cycle<\/h1>\n<\/div>\n
The events of the menstrual cycle<\/a> that take place in the uterus make up the\u00a0uterine cycle<\/a><\/strong>. This cycle consists of changes that occur mainly in the endometrium<\/a>, which is the layer of tissue that lines the uterus. The uterine cycle is divided into the following three phases: menstruation, proliferative phase, and secretory phase. These phases are illustrated in Figure 18.8.4.<\/p>\n
Figure 18.8.4 The uterine cycle begins with menstruation, which starts on day 1 of the cycle. The relative thickness of the endometrium in each phase is indicated in pink.<\/em><\/figcaption><\/figure>\n Menstruation<\/h2>\n
Menstruation<\/a><\/strong> (also called menstrual period or menses) is the first phase of the uterine cycle. It occurs if fertilization<\/a> has not taken place during the preceding menstrual cycle. During menstruation, the endometrium<\/a> of the uterus, which has built up during the preceding cycle, degenerates and is shed from the uterus<\/a>, flowing through an opening in the cervix, and out through the external opening of the vagina. The average loss of blood during menstruation is about 35 mL (about 1 oz or 2 tablespoons). The flow of blood is often accompanied by uterine cramps, which may be severe in some women.<\/p>\n
Proliferative Phase<\/h2>\n
The\u00a0proliferative phase<\/a><\/strong>\u00a0is the second phase of the uterine cycle. During this phase, estrogen<\/a> secreted by\u00a0cells\u00a0of the maturing ovarian follicle<\/a> causes the lining of the uterus to grow, or proliferate. Estrogen also stimulates the cervix<\/a> of the uterus to secrete larger amounts of thinner mucus that can help\u00a0sperm<\/a>\u00a0swim through the cervix and into the uterus, making fertilization more likely.<\/p>\n
Secretory Phase<\/h2>\n
The\u00a0secretory phase<\/a><\/strong>\u00a0is the third and final phase of the uterine cycle<\/a>. During this phase, progesterone<\/a> produced by the corpus luteum<\/a> in the ovary stimulates further changes in the endometrium<\/a> so it is more receptive to implantation of a blastocyst<\/a>. For example, progesterone increases blood flow to the uterus and promotes uterine secretions. It also decreases the contractility of smooth muscle<\/a> tissue in the uterine wall.<\/p>\n
<\/p>\n
Bringing it All Together<\/h1>\n
It is important to note that the pituitary gland, the ovaries and the uterus are all responsible for parts of the ovarian and uterine cycles.\u00a0 The pituitary hormones, LH and FSH affect the ovarian cycle and its hormones.\u00a0 The ovarian hormones, estrogen and progesterone affect the uterine cycle and also feedback on the pituitary gland.\u00a0 Look at Figure 18.8.5 and look at what is happening on different days of the cycle in each of the sets of hormones, the ovarian cycle and the uterine cycle.<\/p>\n
Figure 18.8.5 The pituitary gland, ovarian cycle and uterine cycle are all interrelated in their regulation and participation in the monthly reproductive cycles of women.<\/em><\/figcaption><\/figure>\n <\/p>\n
\n\n\n 18.8 Summary<\/span><\/h1>\n<\/header>\n
\n- \n
- The menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries<\/a> and uterus, and is controlled by pituitary gland<\/a> hormones (FSH<\/a> and LH<\/a>) and ovarian hormones (estrogen<\/a> and progesterone<\/a>).<\/li>\n
- The female reproductive period is delineated by menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n
- The events of the menstrual cycle that take place in the ovaries make up the ovarian cycle<\/a>. It includes the follicular phase<\/a> (when a follicle<\/a> and its ovum mature due to rising levels of FSH), ovulation<\/a> (when the ovum<\/a> is released from the ovary due to a rise in estrogen and a surge in LH), and the luteal phase<\/a> (when the follicle is transformed into a structure called a corpus luteum that secretes progesterone). In a 28-day menstrual cycle, the follicular and luteal phases typically average about two weeks in length, with ovulation generally occurring around day 14 of the cycle.<\/li>\n
- The events of the menstrual cycle<\/a> that take place in the uterus<\/a> make up the uterine cycle. It includes menstruation<\/a>, which generally occurs on days 1 to 5 of the cycle and involves shedding of endometrial tissue that built up during the preceding cycle; the proliferative phase<\/a>, during which the endometrium builds up again until ovulation<\/a> occurs; and the secretory phase<\/a>, which follows ovulation and during which the endometrium secretes substances and undergoes other changes that prepare it to receive an embryo<\/a>.<\/li>\n<\/ul>\n<\/div>\n<\/div>\n
\n\n 18.8 Review Questions<\/span><\/h1>\n<\/header>\n
\n- \n
- \n\n
- The female reproductive period is delineated by menarche<\/a>, or the first menstrual period, which usually occurs around age 12 or 13; and by menopause<\/a>, or the cessation of menstrual periods, which typically occurs around age 52. A typical menstrual cycle averages 28 days in length but may vary normally from 21 to 45 days. The average menstrual period is five days long, but may vary normally from two to seven days. These variations in the menstrual cycle may occur both between women and within individual women from month to month.<\/li>\n
- The menstrual cycle<\/a> refers to natural changes that occur in the female reproductive system each month during the reproductive years, except when a woman is pregnant. The cycle is necessary for the production of ova and the preparation of the uterus<\/a> for pregnancy<\/a>. It involves changes in both the ovaries<\/a> and uterus, and is controlled by pituitary gland<\/a> hormones (FSH<\/a> and LH<\/a>) and ovarian hormones (estrogen<\/a> and progesterone<\/a>).<\/li>\n
![WaterforPeople_Uganda<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:WaterforPeople_Uganda.jpg\"){kind=link}
![Ovarian Cycle<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_43_04_04.png%20andhttps:\/\/commons.wikimedia.org\/wiki\/File:MenstrualCycle2_en.svg\"){kind=link}
![Figure_43_04_04<\/a>\u00a0by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_43_04_04.png\"){kind=link}
![Ovarian and menstrual cycle<\/a>\u00a0by](\"https:\/\/commons.wikimedia.org\/wiki\/File:Figure_28_02_07.jpg\"){kind=link}
![1000px-MenstrualCycle2_en.svg<\/a> by](\"https:\/\/commons.wikimedia.org\/wiki\/File:MenstrualCycle2_en.svg\"){kind=link}