menstruation, Periodic discharge from the vagina of blood, secretions, and shed mucous lining of the uterus (endometrium). The endometrium prepares to receive a fertilized egg by thickening and producing secretions. If the egg released by the ovary is not fertilized, the endometrium breaks down and is expelled by contractions of the uterus. The first menstruation (menarche) occurs after other changes of puberty, usually at 11–13 years of age, apparently triggered by the passing of a weight threshold. Bleeding may be irregular or heavy at first. In adult women, menstrual periods begin at an average interval of 28 days and last about five days; some variation among women and in the same woman is normal. Uterine contractions are felt as cramps. The amount of blood lost is usually less than 1.7 oz (50 ml). Menstruation ends with menopause. Menstrual disorders include dysmenorrhea (painful menstruation) and amenorrhea (no bleeding), heavy or light bleeding, and uterine bleeding. See also premenstrual syndrome.
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