The periodic sloughing off of the uterine lining (endometrium) during a woman*s reproductive years.

Approximately once a month, women of childbearing age experience a bloody vaginal discharge known as menstruation. The onset of menstruation (menarche) usually occurs sometime between 11 and 15 years of age (average age is 12.8). The average menstrual cycle lasts approximately 28 days, although anywhere between 20 and 36 days is considered normal. (The menstrual cycle is measured from the first day of one period to the first day of the following period.) Monthly menstruation continues until a woman reaches menopause, which ordinarily occurs between the ages of 45 and 50.

Menstruation is the result of complex hormonal interactions. During the typical menstrual cycle, the ovaries produce estrogen and progesterone, which cause ovulation to occur and the lining of the uterus (endometrium) to build up. If sexual intercourse takes place and a sperm fertilizes the egg (ovum) as it moves through...