Menstrual Cycle
MENSTRUAL CYCLE
The menstrual cycle encompasses approximately four weeks framed by two menstrual flows (called "periods"). Though few population-based, hormonally valid prospective studies of menstrual cycle intervals and ovulation are available, normal menstrual cycles are twenty-one to thirty-five days long with flow lasting three to five days. The menstrual cycle occurs during approximately thirty to forty-five years of a woman's life beginning with menarche (the first flow) at ages ten to sixteen. The menstrual cycles permanently end with menopause (one year following the final menstrual period), which occurs between ages forty and fifty-eight.
Within each normal menstrual cycle a complex, highly coordinated series of hormonal, physiological and physical changes occur in a predictable fashion. The cycle is divided by ovulation into two phases called follicular and the luteal phase. The start of flow is cycle day 1. The follicular phase leads to increased sexual interest at midcycle, slippery (like egg white) cervical mucous, and release of an egg (ovulation). Ovulation marks the end of the follicular and start of the luteal phase that itself ends with flow. Luteal phase length is ten to sixteen days, during which changes occur in the endometrium (lining of the uterus), breasts, fluid balance, exercise physiology, metabolism, and women's experiences (molimina). If fertilization does not occur, the thickened endometrium starts to shed and a new cycle begins. The normal menstrual flow entails approximately 43 ± 2.3 (median 32) milliliters of blood loss and will soak two to eight regular-sized pads or tampons.
Menstrual interval and ovulatory disturbances (see below) are most common in adolescence (young gynecological age) and in the years prior to menopause (perimenopau). In general, they are reversible and treatable and thus represent disturbances of physiology rather than diseases.
DISTURBANCES OF MENSTRUAL FLOW
Menorrhagia, abnormally heavy flow, occurs at the extremes of menstrual life when ovulation disturbances are also common. Women older than forty-five or fifty tend to have greater blood loss with more variability than women of other ages. The cause of menorrhagia is often unclear but it entails soaking over eleven to sixteen pads or tampons and is associated with clots, cramping (dysmenorrhea), and anemia.
DISTURBANCES OF CYCLE INTERVAL
Amenorrhea, no vaginal bleeding for six or more months, indicates a rare anatomical abnormality (of uterus or vagina), very low or noncyclic, normal estrogen production. Primary amenorrhea means delay of menarche beyond fifteen years of age in 6.4 percent of the population.
Secondary amenorrhea, after menarche, is rare—it occurs in about 1 to 2 percent of the population. The most common causes are (undiagnosed) pregnancy, lactation, young gynecological age (years after menarche), undernutrition or weight loss, and emotional stress (including depression, anxiety, and eating disorders [anorexia and bulemia]). Although amenorrhea is attributed to exercise, it is more likely related to coexistent emotional stress, nutritional deficiencies, and young age.
Oligomenorrhea, flow at intervals longer than thirty-six (but less than 180) days, is more common than amenorrhea and also occurs at the extremes of reproductive life. However, 30 percent of women twenty to forty-nine years old had cycle intervals
Figure 1
over sixty days. Women reporting a body mass index at age eighteen that was over twenty-four had increasing risks for oligomenorrhea with increasing weight.
Polyemnorrhea, (short cycles) are under twenty-one days in length, are common at extremes of reproductive life, and imply higher estrogen production. Short cycles are commonly abnormal in ovulatory characteristics and often have increased in flow.
DISTURBANCES OF OVULATION
Ovulatory disturbances are of two main types: low hypothalamic/pituitary stimulation, called "hypothalamic" or high pituitary stimulation called "anovulatory androgen excess." Ovulatory disturbances of either type include anovulation and cycles with ovulation but short luteal phase length. Anovulation (lack of egg release) universally causes ovarian cysts.
Hypothalamic ovulatory disturbances are common but not often detected because they occur in "regular" cycles of normal interval and flow. Hypothalamic ovulatory disturbances explain approximately 25 percent of infertility and 20 percent of prospectively documented cancellous bone loss. Seventy-five percent of normal weight, healthy premenopausal women experienced at least one cycle with ovulatory disturbance during one-year prospective monitoring, thus this may be an unrecognized cause for osteoporosis. Although not all investigators agree, no other prospective one-year study has simultaneously and continuously documented both ovulation and bone loss.
Hypothalamic ovulatory disturbances are related to cortisol excess caused by physical or psychological stress including cognitive dietary restraint in normal weight women. Ovulatory disturbances may also be associated with menorrhagia and increased risk for anemia, endometrial cancer, breast swelling, nodularity and/or pain (fibrocystic) problems, troublesome premenstrual symptoms, and breast cancer.
Anovulatory androgen excess (commonly called "polycystic ovarian disease") occurs in approximately 5 percent of reproductive-age women. This may cause cycle or flow disturbances, acne, or unwanted male-pattern hair changes (increased facial and body hair and head hair loss). This type of anovulation may be related to insulin excess/resistance, gynecological age, and heredity. Health outcomes related to prolonged anovulatory androgen excess include increased risks of endometrial and breast cancers and probable cardiovascular disease (abnormal lipids, central obesity, increased waist/hip/ratio, and insulin resistance) but protection from osteoporosis.
OVERVIEW OF MENSTRUAL CYCLE AND OVULATORY DISTURBANCES
Cycle interval and ovulatory disturbances are common in adolescence and perimenopause. The majority are reversible (except in perimenopause). Treatment with cyclic progesterone is physiological and increases bone mineral and thus minimizes osteoporosis (see Figure 1). Population-based, prospective studies of menstrual cycles, ovulatory characteristics, and health parameters are needed.
Jerilynn C. Prior
(see also: Anorexia; Contraception; Endocrine Disruptors; Fecundity and Fertility; Nutrition; Reproduction; Sports Medicine; Women's Health )
Bibliography
Barr, S. I.; Janelle, K. C.; and Prior, J. C. (1994). "Vegetarian Versus Nonvegetarian Diets, Dietary Restraint, and Subclinical Ovulatory Disturbances: Prospective Six Month Study." American Journal Clinical Nutrition 60:887–894.
Coulam, C. B.; Annegers, J. F; and Kranz, J. S. (1983). "Chronic Anovulation Syndrome and Associated Neoplasia." Obstetrics Gynecology 61:403–407.
Hallberg, L.; Hogdahl, A. M.; Nillson, L.; and Rybo, G. (1966). "Menstrual Blood Loss: A Population Study." Acta Obstetrics and Gynecology Scandinavia. 45:330–351.
Landgren, B. M.; Unden, A. L.; and Diczfalusy, E. (1980). "Hormonal Profile of the Cycle in 68 Normally Menstruating Women." Acta Endocrinology Copenhagen 94:89–98.
Prior, J. C.; Vigna, Y. M.; Schechter, M. T.; and Burgess, A. E. (1990). "Spinal Bone Loss and Ovulatory Disturbances." New England Journal of Medicine 323:1221–1227.
Prior, J. C.; Vigna, Y. M.; Shulzer, M.; Hall, J. E.; and Bonen, A. (1990). "Determination of Luteal Phase Length by Quantitative Basal Temperature Methods: Validation Against the Midcycle LH Peak." Clinical & Investigative Medicine 45:377–392.
Ramcharan, S.; Love, E. J.; Frick, G. H.; and Goldfien, A. (1992). "The Epidemiology of Premenstrual Symptoms in a Population-Based Sample of 2,650 Urban Women: Attributable Risk and Risk Factors." Journal of Clinical Epidemiology 45:377–392.
Rich-Edwards, J. W.; Goldman, M. B.; Willett, W. C.; Hunter, D. J.; Stampfer, M. J.; Colditz, G. A. and Manson, J. E. (1994). "Adolescent Body Mass Index and Infertility Caused by Ovulatory Disorder." American Journal of Obstetrics and Gynecology 171:171–177.
Treloar, A. E.; Boyton, R. E.; Behn, B. G.; and Brown, B. W. (1967). "Variations of the Human Menstrual Cycle through Reproductive Life." International Journal of Fertility 9:77–126.
Vollman, R. F. (1977). Major Problems in Obstetrics and Gynecology, Vol. 7. Toronto: Saunders.
Menstrual Cycle
Menstrual Cycle
The menstrual cycle technically refers to the cyclic changes that take place in the lining of the human uterus over the course of approximately 28 days in adult females. These cycle changes are associated with cyclic changes in the ovaries and in the brain and ovarian hormones. The term menstrual comes from the Latin word menses, meaning month. The purpose of the cyclic changes is to prepare the uterine lining, called the endometrium, to receive a fertilized egg (the zygote). In response to hormone levels, the endometrium thickens as a result of increases in the cells and blood vessels. If fertilized action does not occur, the uterine lining breaks down. The blood, mucus, and pieces of tissue of the thickened endometrial lining are sloughed off through the cervix of the uterus and out of the vagina, in a process called menstruation.
The first phase of the menstrual cycle in the uterus is the proliferative phase, which is followed by the secretory phase, and then by menstruation. Cyclic changes in hormonal levels control and orchestrate the events of the menstrual cycle.
Proliferative phase
During the proliferative phase in the uterus, the wall of the endometrium begins to thicken. This phase of the uterus begins at the end of menstruation and lasts until ovulation, when the egg is ejected from the ovary. Follicle-stimulating hormone (FSH), secreted by the anterior pituitary gland in the brain, targets the ovaries and triggers the maturation process of up to 25 follicles. Each month, only one egg is brought to maturity and is ejected from the Graafian follicle. About 24 hours before ovulation, the pituitary gland releases a surge of a second hormone, luteinizing hormone (LH), which stimulates the release of the egg out of the ovary and into the Fallopian tube.
During the proliferative phase in the uterus, the hormone estrogen is released from the maturing Graafian follicles in the ovaries. Estrogen stimulates the proliferation of cells in the endometrium of the uterus. Estrogen also plays a role in regulating the release of FSH and LH from the pituitary gland. The increasing levels of estrogen in the bloodstream stimulate the secretion of FSH and LH from the anterior pituitary. The increased levels of FSH and LH in turn further increase estrogen secretion from the follicles in the ovaries.
Secretory phase
During the secretory phase of the uterus, the hormone progesterone is produced by the ovaries. Progesterone (as well as estrogen) is secreted by the corpus luteum, (which means yellow body ), which develops from the Graafian follicle. Progesterone secreted by the corpus luteum stimulates the further build-up of the cells in the endometrium of the uterus. Progesterone also stimulates the glands in the uterus to secrete substances that maintain the endometrium and keep it from breaking down. For this reason, this phase of the menstrual cycle is called the secretory phase.
The presence of estrogen and progesterone in the blood inhibit the production of FSH and LH from the pituitary, and the levels of FSH and LH begin to fall.
If the egg that has been ovulated into the Fallopian tube is fertilized by spermatozoa, the developing zygote implants in the thickened endometrium of the uterus approximately seven days after ovulation. This stimulates the endometrium to secrete a human chorionic gonadotropic hormone (HCG). HCG maintains the corpus luteum in the ovary, so that it continues to secrete progesterone. HCG is secreted throughout pregnancy and keeps blood progesterone levels high, so that the endometrium continues to thicken, eventually forming the placenta. Without a high level of progesterone, the endometrium begins to break down. In a pregnancy, the breakdown of the endometrium would result in a miscarriage.
If fertilization does not occur, the corpus luteum shrinks and blood progesterone levels drop, at about day 22 in a 28 day cycle. Without progesterone, the
KEY TERMS
Cervix —The front portion, or neck, of the uterus.
Corpus luteum —The site on the ovary from where the ovum was released; the corpus luteum then releases hormones to prepare the endometrium for implantation of the fertilized ovum.
Endometrium —The blood-rich interior lining of the uterus.
Estrogen —A female reproductive hormone secreted by the follicles.
Follicle —The structure in which eggs develop within the ovary.
Follicle-stimulating hormone (FSH) —A hormone released by the pituitary gland that stimulates ovum production and maturation.
Graafian follicles —Maturing ovarian follicles.
Human chorionic gonadotropin —Hormone secreted by the embryo that maintains the corpus luteum.
Luteinizing hormone —Hormone that acts with LH to stimulate the maturation of follicles.
Menarche —First menstrual period.
Menopause —Cessation of the menstrual cycle.
Menstruation —Sloughing off the lining of the uterus.
Ovary —Female reproductive organ that contains the eggs.
Ovulation —Process in which an egg is ejected from an ovarian follicle.
Progesterone —Hormone secreted by the corpus luteum; maintains the endometrium.
Prostaglandins —Complex fatty acids occurring in most human tissues.
Uterus —Organ in female mammals in which embryo and fetus grow to maturity.
Vagina —Passage from the uterus to outside the female body.
endometrium degenerates, and is expelled through the cervix and out through the vagina.
Menstruation
The expulsion of tissue and blood from the uterus lasts from three to eight days, with much variation among women. Some women experience painful cramps during menstruation, which are the result of uterine contractions that expel the endometrium. Hormones known as prostaglandins are produced by uterine cells during menstruation, and the excessive production of prostaglandins is associated with stronger uterine contractions and more intense cramps. Menstrual cramps can be treated by drugs that inhibit the production of prostaglandins in uterine cells. Aspirin, ibuprofin, and naproxin sodium are all effective anti-prostaglandin drugs. It is important to take these medications at the onset of the menstrual flow, otherwise prostaglandin production can proceed for several hours unchecked, and the drugs will not be effective in reducing pain.
Some women also experience premenstrual syndrome (PMS), a condition occurring some time in the secretory phase prior to menstruation. Symptoms of PMS include mood changes, water retention and bloating, increase in appetite and cravings, cramps, breast pain, and headaches. Researchers are not sure what causes PMS, but the sharp drop in progesterone that occurs at about day 22 may be involved in triggering these physical and emotional symptoms. While no cure for PMS exists, experts recommend that women who experience PMS reduce their salt intake, engage in more exercise, and maintain a healthy diet during this time. The B vitamins may also be effective in reducing PMS symptoms. Some women have found relief in taking the medications prescribed for menstrual cramps, while lowering caffeine intake can be useful in reducing premenstrual breast pain.
Girls begin menstruating at the onset of puberty, at about the age of 12 or 13 years, although the onset of menstruation may be earlier or later, depending on the amount of body fat. The first menstrual period is called the menarche; during the first few cycles, ovulation may be absent.
Menopause is the cessation of the menstrual cycle, when ovulation and menstruation cease. The cessation of menstruation is gradual and is preceded by menstrual cycles in which ovulation does not occur. The menstrual cycle becomes irregular before finally stopping completely. The onset of menopause is individually variable, occurring between the ages of 45 to the late fifties.
Resources
BOOKS
Kumar, Anand, and A.K. Mukhopadhvay, eds. Follicular Growth, Ovulation, and Fertilization. New Delhi, India: Narosa, 2002.
Northrup, Christiane. The Wisdom of Menopause: Creating Physical and Emotional Health and Healing During the Change. New York: Bantam Books, 2003.
Sloane, Ethel. Biology of Women. 4td ed. Albany, NY: Delmar/Thomson Learning, 2002.
Stewart, Donna E., ed. Menopause: A Mental Health Practitioner’s Guide. Washington, DC: American Psychiatric Pub., 2005.
Kathleen Scogna
Menstrual Cycle
Menstrual cycle
The menstrual cycle technically refers to the cyclic changes that take place in the lining of the human uterus over the course of approximately 28 days in adult females. These cycle changes are associated with cyclic changes in the ovaries and in the brain and ovarian hormones . The term "menstrual" comes from the Latin word menses, meaning month. The purpose of the cyclic changes is to prepare the uterine lining, called the endometrium, to receive a fertilized egg (the zygote). In response to hormone levels, the endometrium thickens as a result of increases in the cells and blood vessels. If fertilized action does not occur, the uterine lining breaks down. The blood, mucus, and pieces of tissue of the thickened endometrial lining are sloughed off through the cervix of the uterus and out of the vagina, in a process called menstruation.
The first phase of the menstrual cycle in the uterus is the proliferative phase, which is followed by the secretory phase, and then by menstruation. Cyclic changes in hormonal levels control and orchestrate the events of the menstrual cycle.
Proliferative phase
During the proliferative phase in the uterus, the wall of the endometrium begins to thicken. This phase of the uterus begins at the end of menstruation and lasts until ovulation, when the egg is ejected from the ovary. Follicle-stimulating hormone (FSH), secreted by the anterior pituitary gland in the brain, targets the ovaries and triggers the maturation process of up to 25 follicles. Each month, only one egg is brought to maturity and is ejected from the Graafian follicle. About 24 hours before ovulation, the pituitary gland releases a surge of a second hormone, luteinizing hormone (LH), which stimulates the release of the egg out of the ovary and into the Fallopian tube.
During the proliferative phase in the uterus, the hormone estrogen is released from the maturing Graafian follicles in the ovaries. Estrogen stimulates the proliferation of cells in the endometrium of the uterus. Estrogen also plays a role in regulating the release of FSH and LH from the pituitary gland. The increasing levels of estrogen in the bloodstream stimulate the secretion of FSH and LH from the anterior pituitary. The increased levels of FSH and LH in turn further increase estrogen secretion from the follicles in the ovaries.
Secretory phase
During the secretory phase of the uterus, the hormone progesterone is produced by the ovaries. Progesterone (as well as estrogen) is secreted by the corpus luteum, (which means yellow body), which develops from the Graafian follicle. Progesterone secreted by the corpus luteum stimulates the further build-up of the cells in the endometrium of the uterus. Progesterone also stimulates the glands in the uterus to secrete substances that maintain the endometrium and keep it from breaking down. For this reason, this phase of the menstrual cycle is called the secretory phase.
The presence of estrogen and progesterone in the blood inhibit the production of FSH and LH from the pituitary, and the levels of FSH and LH begin to fall.
If the egg that has been ovulated into the Fallopian tube is fertilized by spermatozoa, the developing zygote implants in the thickened endometrium of the uterus approximately seven days after ovulation. This stimulates the endometrium to secrete a human chorionic gonadotropic hormone (HCG). HCG maintains the corpus luteum in the ovary, so that it continues to secrete progesterone. HCG is secreted throughout pregnancy and keeps blood progesterone levels high, so that the endometrium continues to thicken, eventually forming the placenta. Without a high level of progesterone, the endometrium begins to break down. In a pregnancy, the breakdown of the endometrium would result in a miscarriage.
If fertilization does not occur, the corpus luteum shrinks and blood progesterone levels drop, at about day 22 in a 28 day cycle. Without progesterone, the endometrium degenerates, and is expelled through the cervix and out through the vagina.
Menstruation
The expulsion of tissue and blood from the uterus lasts from three to eight days, with much variation among women. Some women experience painful cramps during menstruation, which are the result of uterine contractions that expel the endometrium. Hormones known as prostaglandins are produced by uterine cells during menstruation, and the excessive production of prostaglandins is associated with stronger uterine contractions and more intense cramps. Menstrual cramps can be treated by drugs that inhibit the production of prostaglandins in uterine cells. Aspirin, ibuprofin, and naproxin sodium are all effective anti-prostaglandin drugs. It is important to take these medications at the onset of the menstrual flow, otherwise prostaglandin production can proceed for several hours unchecked, and the drugs will not be effective in reducing pain .
Some women also experience premenstrual syndrome (PMS), a condition occurring some time in the secretory phase prior to menstruation. Symptoms of PMS include mood changes, water retention and bloating, increase in appetite and cravings, cramps, breast pain, and headaches. Researchers are not sure what causes PMS, but the sharp drop in progesterone that occurs at about day 22 may be involved in triggering these physical and emotional symptoms. While no cure for PMS exists, experts recommend that women who experience PMS reduce their salt intake, engage in more exercise , and maintain a healthy diet during this time. The B vitamins may also be effective in reducing PMS symptoms. Some women have found relief in taking the medications prescribed for menstrual cramps, while lowering caffeine intake can be useful in reducing premenstrual breast pain.
Girls begin menstruating at the onset of puberty , at about the age of 12 or 13, although the onset of menstruation may be earlier or later, depending on the amount of body fat . The first menstrual period is called the menarche; during the first few cycles, ovulation may be absent.
Menopause is the cessation of the menstrual cycle, when ovulation and menstruation cease. The cessation of menstruation is gradual and is preceded by menstrual cycles in which ovulation does not occur. The menstrual cycle becomes irregular before finally stopping completely. The onset of menopause is individually variable, occurring between the ages of 45 to the late fifties.
Resources
books
Ferin, Michel, et al. The Menstrual Cycle: Physiology, Reproductive Disorders, and Infertility. New York: Oxford University Press, 1993.
Sloane, Ethel. Biology of Women. 3rd ed. Albany, NY: Delmar, 1993.
periodicals
Mestel, Rosie. "Are Periods a Protection Against Men?" New Scientist 140, no. 1893 (October 2, 1993): 8.
Quadagno, David, et al. "The Menstrual Cycle: Does It Affect Athletic Performance?" The Physician and Sportsmedicine 19, no. 3 (March 1991): 121.
Segal, Marion. "A Balanced Look at the Menstrual Cycle." FDA Consumer 27, no. 1 (December 1993): 32.
Kathleen Scogna
KEY TERMS
- Cervix
—The front portion, or neck, of the uterus.
- Corpus luteum
—The site on the ovary from where the ovum was released; the corpus luteum then releases hormones to prepare the endometrium for implantation of the fertilized ovum.
- Endometrium
—The blood-rich interior lining of the uterus.
- Estrogen
—A female reproductive hormone secreted by the follicles.
- Follicle
—The structure in which eggs develop within the ovary.
- Follicle-stimulating hormone (FSH)
—A hormone released by the pituitary gland that stimulates ovum production and maturation.
- Graafian follicles
—Maturing ovarian follicles.
- Human chorionic gonadotropin
—Hormone secreted by the embryo that maintains the corpus luteum.
- Luteinizing hormone
—Hormone that acts with LH to stimulate the maturation of follicles.
- Menarche
—First menstrual period.
- Menopause
—Cessation of the menstrual cycle.
- Menstruation
—Sloughing off the lining of the uterus.
- Ovary
—Female reproductive organ that contains the eggs.
- Ovulation
—Process in which an egg is ejected from an ovarian follicle.
- Progesterone
—Hormone secreted by the corpus luteum; maintains the endometrium.
- Prostaglandins
—Complex fatty acids occurring in most human tissues.
- Uterus
—Organ in female mammals in which embryo and fetus grow to maturity.
- Vagina
—Passage from the uterus to outside the female body.
menstrual cycle
The first day of the menstrual cycle is defined as the first day of menstrual blood loss. This is when the uterus begins to shed its lining and bleeding occurs. At this time the secretion of hormones (oestrogen and progesterone) from the ovaries is at a minimum. This diminishes the braking effect that circulating ovarian hormones have on the secretion of the gonadotrophic hormones from the pituitary gland, namely luteinizing hormone (LH) amd follicle stimulating hormone (FSH). As a consequence these pituitary secretions increase and stimulate a new wave of activity in the ovaries.
Early in the cycle, FSH stimulates growth of a few follicles (egg-containing ‘sacs’) in each ovary. By about day 10 the ovaries contain several follicles with a diameter of 14–21 mm. As mid cycle approaches, all but one of these degenerate, and only the ‘dominant’ follicle becomes fully mature, with a diameter of 20–25 mm. What determines which follicle becomes the dominant one, and in which ovary, remains speculative. Local hormones or other factors acting within the ovaries may play an important role. This first half of the ovarian cycle is known as the follicular phase and is characterized by increasing secretions of oestrogen from the developing follicles; this is released into the bloodstream, reaches the uterus, and causes its lining to thicken: the glands enlarge and it becomes richly supplied with blood vessels: the proliferative phase of the uterine cycle.
In most normal human menstrual cycles only one follicle reaches full maturity, to be released at ovulation, on about day 14. The occasional release of two accounts for non-identical twins, and fertility drugs can increase the number of follicles reaching maturity at mid cycle. These drugs are either pituitary gonadotrophins, or synthetic chemicals which interfere with the negative feedback loop in such a way as to promote an increase in the release of these hormones from the pituitary gland itself. In both cases the ovaries receive an increased ‘drive’ for follicular development, and thus several follicles will mature. Such drugs are used for treating certain types of infertility, and are given to women undergoing in vitro fertilization (IVF) treatment. If the result is multiple ovulation, the chances of fertilization are increased or, in the case of IVF, more than one mature egg can be recovered for external fertilization and subsequent implantation.
At mid cycle there is a dramatic change of events. There is a high blood concentration of oestrogen, but this ceases to have a braking (negative feedback) effect on the pituitary hormones. About 24–48 hours after the peak of oestrogen production a surge of the gonadotrophins occurs — especially of luteinizing hormone. This is one of the rare biological examples of a positive feedback action. The surge causes the mature ‘dominant’ follicle to rupture and release its egg within 9–12 hours. Indeed, one way of predicting ovulation is by the detection of the increase in luteinizing hormone in the blood, which is reflected in the urine. This is the scientific basis for the kits which are commercially available to identify the most likely time for conception.
At the time of ovulation there is a small rise in body temperature. This is thought to be due to the action of rising progesterone in the blood, resetting in some way the ‘thermostat’ in the brain which controls our body temperature. This small rise can be used to indicate when ovulation occurs, but obtaining reliable temperature measurements is difficult, making the method often unsatisfactory. Some women feel mild pain in the abdomen around the time of ovulation, lasting from a few minutes to a couple of hours. Known as Mittelschmerz (German for ‘midpain’), it is probably caused by irritation of the abdominal wall due to blood and fluid escaping from the ruptured follicle. Changes in the cervical mucus also occur about the time of ovulation.
After ovulation the empty follicle left behind in the ovary is remodelled, and it plays an important role in the second half of the menstrual cycle, known as the luteal phase of the ovarian cycle. The cells remaining in the ruptured follicle proliferate rapidly and form the corpus luteum. This ‘yellow body’ produces increasing amounts of progesterone and some oestrogen, and these hormones act on the lining of the womb — it becomes thick and spongy and its glands secrete nutrients that can be used by the embryo if fertilization has occurred: this is the secretory phase of the uterine cycle. The high progesterone level in the blood, together with oestrogen, also exerts negative feedback effects, which decrease the secretion of the gonadotrophin-promoting secretion by which the hypothalamus influences the pituitary. Small amounts of gonadotrophins nevertheless continue to maintain the function of the corpus luteum — but if fertilization does not occur, towards the end of the cycle this support fails and the corpus luteum breaks down. The precise mechanisms which induce this degeneration are unknown, but the consequences are that progesterone and oestrogen secretions decline, the hormonal support of the uterine lining is lost, the spiral arteries contract, and the lining cells, starved of their blood supply, break away. Menstrual bleeding ensues. A new cycle begins.
While the average time for each menstrual cycle is typically depicted as 28 days, cycles do vary considerably in length, ranging from 25 days to 35 days. It is usually the length of the first (follicular) phase of the cycle that accounts for most of the variation. The luteal phase is more likely to last the typical 14 days, with ovulation occurring two weeks before rather than after the onset of menstruation, so it is unpredictable. Furthermore, the luteal phase in some women can also vary. This variability clearly makes ‘safe period’ birth control unreliable.
Saffron Whitehead
Bibliography
Jones, R. E. (1997). Human reproductive biology, (2nd edn). Academic Press, New York.
See also menstruation; premenstrual tension; ova; ovary; sex hormones; uterus.
menstrual cycle
Menstrual Cycle
MENSTRUAL CYCLE
The menstrual cycle is a periodic flow of blood and cells from the lining of the uterus in human females and the females of most other primates, occurring about every twenty-eight days. The beginning of menstruation, or menarche (the first menstrual period), typically starts between the ages of ten and seventeen and is a sign of readiness for childbearing.
During each cycle, the lining, or endometrium, of the uterus experiences a rapid generation of cells and vein-filled channels in preparation for pregnancy. Halfway through the cycle, an ovum (egg) is released from an ovary. The ovum passes through the fallopian tube, and if fertilized by a sperm, the ovum is implanted in the uterus, and the thickened lining helps support the pregnancy. If the ovum is not fertilized, the tissue and blood are shed.
The many myths and taboos related to menstruation have caused some cultures to chastise it as "un-clean" or a "curse." For a young girl, menarche is simply related to growth and body weight. Signs of puberty can begin after the age of eight, but early physical maturation may result in social pressure because of increased attention.
See also:ADOLESCENCE; CONTRACEPTION; MENARCHE; PUBERTY
Bibliography
Gorman, Christine. "Growing Pains: What Happens When Puberty Comes Too Soon in Your Child—and What You Can Do about It." Time (August 21, 2000):84.
Peters, Diane. "It's Wonderful Being a Girl." Chatelaine (June2000):76.
Beth A.Kapes