月经(menstruation)
妇女周期性地从阴道排出血液、黏液和脱落的黏液性子宫内壁组织（子宫内膜）的生理现象。子宫内膜增厚和制造分泌物是为孕卵着床而准备。卵巢释放出的卵若未受精，子宫内膜组织会脱落并经子宫收缩而排出。月经第一次来潮（初潮）出现於青春期其他变化之後，通常为十一～十三岁时，明显地是因通过一重量上的临界点而触发。初潮前已出现第二性徵，初潮後的数次月经周期可能不规律。若至十六岁仍不来月经则应作妇科检查。月经初期，其周期可能不规律或是经血量较多。成年妇女的月经周期平均约二十八天，月经期持续约五天。不同妇女或同一个妇女各周期长短略异为正常现象。某些女性会因子宫收缩而有痛性痉挛。每次来潮血量通常少於50公撮。月经因停经期出现而结束。经期障碍包括痛经（月经来时疼痛）和闭经（无月经）、经血过多或过少、子宫出血。亦请参阅premenstrual syndrome (PMS)。
English version：
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.