Menstrual blood is drained from the uterus through the cervix to the vagina and out of the body. Menstruation is accompanied by cyclical changes in the ovaries and the cyclical shedding and bleeding of the endometrium. Regular menstrual bleeding is an important sign of mature reproductive function. Menstrual blood is dark red in color and contains, in addition to blood, endometrial debris, cervical mucus, and shed vaginal epithelial cells. Menstrual blood contains prostaglandins and a large amount of fibrinolytic enzymes from the endometrium, so that menstrual blood does not clot, and only clots appear in cases of heavy bleeding. In the case of cervical atresia or hymenal atresia, this can cause menstrual blood not to drain and pain in the lower abdomen, which needs to be treated by surgical means.