Menstrual Cycle (Encyclopedia of Public Health)
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 raret 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
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)
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