MENSTRUATION

periodic discharge from the vagina of blood, secretions, and disintegrating mucous membrane that had lined the uterus. The biological significance of the process in humans can best be explained by reference to the reproductive function in other mammals. In a number of species of wild sheep, for example, there is only one breeding season in the year; during this season a cycle of changes takes place in the reproductive organs, characterized by ripening and release of ova from the ovaries, increased blood supply to the genital tract, growth of the uterus, and proliferation of its lining. There is a discharge of blood and mucus from the uterus and vagina, and this is the time when coition may take place. Pregnancy normally follows, but if the ewe is not served by the ram the changes retrogress until the next breeding season. This cycle of changes is termed the estrous cycle. In many domesticated sheep there is more than one estrous cycle in the breeding season. If the ewe does not become pregnant in the first cycle there is a short resting phase; then ovulation is repeated and another cycle of activity of the reproductive system takes place. After each breeding period, with its succession of estrous cycles, there is a relatively long resting phase. In most female primates, including women, there is no resting phase; an unbroken series of estrous cycles occurs throughout the year, and pregnancy can occur in any one of them. In some animals a variety of external stimuli act through the central nervous system on the hypothalamic region of the brain. The hypothalamus controls the release from the pituitary gland of hormones that induce ripening of ovarian folliclesova and the cellular structures that enclose them. These pituitary hormones, called gonadotropic hormones, are carried to the ovaries by way of the bloodstream. In primates the hypothalamic mechanism normally is independent of external stimuli, and regular discharge of ova into the tubes leading to the uterus occurs even in the absence of coitus. Under the influence of the pituitary gonadotropic hormones, the ovary produces other hormones, which cause growth and increased vascularity of the uterus and vagina. These hormones are estrogenschiefly 17 beta-estradioland progesterone. It is as though the ovary prepares the uterus for the reception of the ovum that is released in the particular cycle. the periodic discharge from the vagina of blood, secretions, and disintegrating tissue that had lined the uterus. Menstruation takes place if the ovum (egg) released by the ovary has not been fertilized. The average human menstrual cycle is 28 days, but cycles ranging from 21 to 35 days are not uncommon. At the beginning of the cycle, the endometrium, or lining of the uterus, is thin, and the ovaries are quiescent. Under the influence of hormones from the pituitary gland, a follicle ripens in one of the ovaries. The follicle contains an ovum, surrounded by a group of cells that secrete an estrogenic hormone. This hormone causes the endometrium to become thicker and more vascular. At about mid-cycle ovulation occurs; the follicle ruptures and discharges the ovum, which enters the fallopian tube and travels to the uterus. After ovulation the cells of the follicle that extruded the ovum form the so-called corpus luteum. This body continues to secrete estrogen and also begins to release progesterone, which induces the secretory phase in the endometrium. The glands and cells of the endometrium become swollen, and the entire uterine lining thickens in preparation for receiving the ovum. If the released ovum becomes fertilized, it embeds itself in the endometrium and begins to grow. If the ovum is not fertilized, it dies, and the ovary ceases to produce hormones, causing a spasm of the endometrial blood vessels and consequent breakdown of the uterine lining. Menstruation then occurs, and the disintegrating endometrium is sloughed off and discharged along with blood. The endometrial tissue regenerates during the proliferative phase of the next cycle. Menstruation ordinarily begins between the ages of 11 and 13. The first menstrual periods may be somewhat erratic, or bleeding may be heavy. Usually regularity is established spontaneously. Each period lasts for about five days, but the duration and amount of flow vary considerably. Before the onset of a period, some women experience pelvic discomfort, soreness of the breasts, emotional tension, or tension due to fluid retention in the tissues. During the period, discomfort or some pain may be felt as the uterus contracts to expel the blood and disintegrating tissue. Menstruation continues on the average until the age of 45 or 50, when ovarian activity gradually declines and then ceases. The menstrual periods may stop abruptly, but often they become irregular and gradually diminish. The final cessation of the cycles, or menopause, marks the end of the reproductive period in a woman's life. Disorders of the menstrual function include dysmenorrhea (painful menstruation); irregular or excessive bleeding due to a hormonal imbalance, disease of the pelvic organs, or emotional disturbance; and amenorrhea (absence of menstruation). Menstrual periods are normally absent throughout pregnancy and for a variable time after delivery. If a mother breast-feeds her baby, menstruation may not recur for as long as six months. Ovulation occurs at about the midpoint of each menstrual cycle, and the ovum is capable of fertilization for about two days after that event. The rhythm method of contraception is based on the fact that for most women the time of ovulation is fairly constant. When cycles are of irregular length, however, the time of ovulation is uncertain and may vary unexpectedly in women with usually regular cycles. Infertility in some women is a consequence of cycles in which there is no ovulation although regular menstruation occurs. Cycles of this type take place in women who use oral contraceptives. Additional reading Information on menstruation may be found in Doreen Asso, The Real Menstrual Cycle (1983); John G. Gruhn and Ralph R. Kazer, Hormonal Regulation of the Menstrual Cycle (1989); and Linda R. Gannon, Menstrual Disorders and Menopause: Biological, Psychological, and Cultural Research (1985). The Editors of the Encyclopdia Britannica