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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 rareit 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 60:887894.

Coulam, C. B.; Annegers, J. F; and Kranz, J. S. (1983). "Chronic Anovulation Syndrome and Associated Neoplasia." Obstetrics Gynecology 61:403407.

Hallberg, L.; Hogdahl, A. M.; Nillson, L.; and Rybo, G. (1966). "Menstrual Blood Loss: A Population Study." Acta Obstetrics and Gynecology Scandinavia. 45:330351.

Landgren, B. M.; Unden, A. L.; and Diczfalusy, E. (1980). "Hormonal Profile of the Cycle in 68 Normally Menstruating Women." Acta Endocrinology Copenhagen 94:8998.

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 323:12211227.

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 45:377392.

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 45:377392.

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 171:171177.

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 9:77126.

Vollman, R. F. (1977). Major Problems in Obstetrics and Gynecology, Vol. 7. Toronto: Saunders.

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menstrual cycle

menstrual cycle Throughout a woman's reproductive life — from puberty to the menopause — the ovaries are programmed to produce a mature egg (ovum) approximately every 28 days and to prepare the uterus (womb) for implantation of an embryo if the egg becomes fertilized. To achieve this reproductive competence the ovaries must receive the correct hormonal signals from the brain and the pituitary gland. These signals stimulate the production of female sex hormones and the cyclical changes which occur in the ovary during each normal menstrual cycle. In turn the sex steroids released by the ovary induce changes in the lining of the womb and other parts of the female reproductive tract. The system is subtly regulated and fine-tuned by feedback effects of the ovarian steroid hormones on hormone secretions from the hypothalamus and pituitary gland, so there is a complex interplay of hormones and feedback signals which ultimately controls female fertility. Collectively these events constitute the 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.

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menstrual cycle

menstrual cycle In humans and some higher primates of reproductive age, the stage during which the body prepares for pregnancy. In humans, the average cycle is 28 days. At the beginning of the cycle, hormones from the pituitary gland stimulate the growth of an ovum contained in a follicle in one of the two ovaries. At approximately mid-cycle, the follicle bursts, the ovum releases (ovulation) and travels down the Fallopian tube to the uterus. The follicle (now called the corpus luteum) secretes two hormones, progesterone and oestrogen, during this secretory phase, and the endometrium thickens, ready to receive the fertilized ovum. If fertilization (conception) does not occur, the corpus luteum degenerates, hormone secretion ceases, the endometrium breaks down, and menstruation occurs in the form of a loss of blood. In the event of conception, the corpus luteum remains and maintains the endometrium with hormones until the placenta is formed. In humans, the onset of the menstrual cycle occurs at puberty; it ceases with the menopause (around 50 years of age). See also ovary

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menstrual cycle

menstrual cycle (sexual cycle) The approximately monthly cycle of events associated with ovulation that replaces the oestrous cycle in most primates (including humans). The lining of the uterus becomes progressively thicker with more blood vessels in preparation for the implantation of a fertilized egg cell (blastocyst). Ovulation occurs during the middle of the cycle (the fertile period). If fertilization does not occur the uterine lining breaks down and is discharged from the body (menstruation); the discharge is known as a `period'. In women the fertile period is 11–15 days after the end of the last menstruation.

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menstrual cycle

menstrual cycle (men-stroo-ăl) n. the periodic sequence of events in sexually mature nonpregnant women by which an egg cell (ovum) is released from a follicle in the ovary at four-weekly intervals until the menopause (see illustration). The secretion of progesterone in the ruptured follicle causes the lining of the uterus (endometrium) to become thicker and richly supplied with blood in preparation for pregnancy. If the ovum is not fertilized the endometrium is shed at menstruation.

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menstrual cycle

men·stru·al cy·cle • n. the process of ovulation and menstruation in women and other female primates.

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menstrual cycle

menstrual cycle The cycle of ovulation that replaces the oestrus cycle in most Primates and follows an approximately monthly rhythm.

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