Menstruation does not come out of the anus, but from the vagina. When a woman enters puberty, the ovaries begin to secrete estrogen and progesterone periodically, and the endometrium changes accordingly under the effect of estrogen and progesterone, and will thicken. When estrogen and progesterone decline, the endometrium temporarily loses the support of estrogen and progesterone, so the endometrium will shed and cause vaginal bleeding, which is clinically known as menstruation. Under normal clinical conditions, the majority of women have a menstrual cycle of 28-30 days, with a clean period of 3-7 days and a menstrual volume of no more than 80 ml.