The shedding of endometrium is periodic, once a month, which is influenced by the periodic changes of ovarian endocrine. Every time the endometrium falls off, the follicles in the ovary begin to develop. Under the influence of estrogen secreted by it, endometrium, including epithelial cells, blood vessels and glands, began to grow and thicken again. The endometrium at this time is called proliferative phase. About the middle of the menstrual cycle, just before the next menstruation 14 days. Ovulation occurs in the oocysts in the ovary (taking the 28-day cycle as an example, 14 is the ovulation day). Follicles after ovulation form corpus luteum, which can secrete estrogen and progesterone. These hormones act on the endometrium, which makes the endometrium thicken, blood vessels bend and congestion, glands thicken and expand, and secrete many nutrients. The endometrium at this time is called secretory phase. These changes are mainly aimed at the nutritional development of fertilized eggs after implantation in endometrium. If the eggs discharged this month are not fertilized, the corpus luteum in the ovary will degenerate, and the estrogen and progesterone produced will decrease. Without the support of estrogen and progesterone, the endometrium will shrink and collapse, and the blood vessels in the endometrium will spasm and contract. Finally, it will rupture and bleed, and the endometrium will be stripped from the uterine cavity and discharged through the vagina. This is menstruation. In women of childbearing age, this transformation is repeated unless they are pregnant, so it is called menstrual cycle. It can also be understood that menstruation is a manifestation of fertilization failure after ovulation once a month.