menstrual cycle The pattern of hormonal and physiologic changes that occur that occur in a woman's body in preparation for possible PREGNANCY. Though the average menstrual cycle spans 28 days, the frequency of MENSTRUATION varies widely among women and often within each woman individually. Menstrual cycles may be as short as 25 days or as long as 32 days and still be within the range of normal. Menstrual cycles outside these parameters may or may not be normal, depending on the woman's individual physiology and health status. The endocrine system directs the menstrual cycle.
Physiologic Phases of the Menstrual Cycle
There are four phases within the menstrual cycle that always occur in the same order:
1. The proliferative phase begins with the end of menstruation and the return of the endometrium (lining of the UTERUS) to its non-menstrual state and culminates with OVULATION about 14 days after the onset of menstruation. During proliferation the level of ESTROGENS in the BLOOD circulation rises and the level of PROGESTERONE drops. The changing hormone levels stimulate the maturation of up to 20 ova within their ovarian follicles, called ripening. The follicle containing the first ovum to reach
full maturity ruptures and releases the ovum into the fluid surrounding the fimbriae (fluted edges) of the fallopian tube. The other follicles that had started to develop then shrink; the ovary absorbs them and their ova.
2. The expelled ovum leaves behind the corpus luteum, a structure of endocrine tissue that begins secreting estrogens and progesterone. This period of activity by the corpus luteum is the luteal phase, also called the secretory phase. The increased blood levels of the hormones cause the endometrium to thicken and its blood vessels to enlarge. The glands that line the endometrium increase their secretions, and the inner endometrium becomes spongy and engorged in preparation to support implantation should CONCEPTION occur. The luteal phase lasts about 10 days.
3. When the ovum passes through the uterus without implanting, the corpus luteum involutes (turns in on itself) and the follicle absorbs it. The sudden drop in estrogens and progesterone causes the endometrial blood vessels to contract, called endometrial ischemia. The endometrial glands stop their secretions and the endometrium dramatically shrinks. This third phase of the menstrual cycle, called the ischemic phase, lasts 36 to 48 hours.
4. The culminating phase of the menstrual cycle is menstruation, during which the anemic (blood-deprived) tissue of the endometrium sloughs away and passes from the body. The menstrual flow contains tissue fragments, endometrial secretions, and blood. Menstruation lasts 3 to 5 days in 85 percent of women; about 15 percent of women menstruate for 7 days. Though menstruation is the last phase of the menstrual cycle, doctors consider the first day of menstrual bleeding to be the start of the menstrual cycle.
Endocrine Regulation of the Menstrual Cycle
The HYPOTHALAMUS, PITUITARY GLAND, and corpus luteum regulate the menstrual cycle. The hypo-thalamus initiates the proliferative phase of the menstrual cycle by releasing GONADOTROPIN-RELEAS-ING HORMONE (GNRH). GnRH stimulates the pituitary gland to secrete a surge of FOLLICLE-STIMULATING
hormone (fsh), which induces an ovarian follicle to begin secreting estrogens. The estrogens cause the ovum within the follicle to begin ripening. The rising level of estrogens in the blood circulation triggers the hypothalamus to again release GnRH, which this time stimulates the pituitary gland to secrete luteinizing hormone (lh). LH causes the ovarian follicle to produce progesterone, which brings the ovum to maturity and release (ovula-tion). Without pregnancy the blood levels of estrogens and progesterone both fall and menstruation takes place.
Disturbances of the Menstrual Cycle
Numerous factors may disrupt the menstrual cycle, the most common being pregnancy. When a fertilized ovum (zygote) implants in the endometrium, the menstrual cycle ends and pregnancy begins. The menstrual cycle does not return until six to eight weeks (and sometimes longer, up to months in women who are breastfeeding) after childbirth. Hormonal imbalances may also disrupt the menstrual cycle. Hypothyroidism (underactive thyroid gland) or hyperthyroidism (overactive thyroid gland) is a common source of such hormonal disruption. Disorders of the pituitary gland, such as pituitary adenoma, or the adrenal glands, such as adrenal insufficiency, often alter the body's endocrine matrix in ways that affect the menstrual cycle.
Numerous medications and treatments such as chemotherapy and radiation therapy may affect ovarian function. Menstrual disturbances may occur as a result of underlying health conditions such as polycystic ovary syndrome (pcos), obesity, anorexia, and extreme emotional or physical stress. Though a normal menstrual cycle often occurs with a single functioning ovary, the absence or loss of both ovaries ends the menstrual cycle. Oophorectomy is the surgical operation to remove an ovary. Women who participate in intense athletic activities, such as marathons and triathlons, may have irregular menstrual cycles or amenorrhea (absence of menstruation).
For further discussion of the menstrual cycle within the context of the structures and functions of reproduction and sexuality, please see the overview section "The Reproductive System." For further discussion of the menstrual cycle within
the context of the structures and functions of the endocrine system, please see the overview section "The Endocrine System."
See also DYSFUNCTIONAL UTERINE BLEEDING (DUB); DYSMENORRHEA; FALLOPIAN TUBES; FERTILITY; INFERTILITY; MENARCHE; MENOPAUSE; PREMATURE OVARIAN FAILURE (POF); PREMENSTRUAL SYNDROME (PMS).
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