Menstruation is a physiological cycle, which occurs between some fertile female humans and female animals such as chimpanzees. Spiny mice, cows, mares, camels, sows, ewes, etc. Menstruation also occurs, and some mammals go through estrus cycle.

In women of childbearing age and primate females, the endometrium undergoes autonomous thickening, vascular proliferation, gland growth and secretion, endometrial collapse and shedding, accompanied by periodic bleeding changes every other month. This periodic vaginal bleeding or uterine bleeding is called menstruation. Menstrual bleeding is also called menstrual blood.

Menstruation is regulated by the interaction of reproductive hormones among hypothalamus, pituitary and ovary. During the menstrual period and proliferation period of menstrual cycle, the levels of estradiol and progesterone in blood are very low, which weakens or eliminates the negative feedback on adenohypophysis and hypothalamus, leading to the increase of gonadotropin-releasing hormone secreted by hypothalamus, which in turn leads to the increase of follicle stimulating hormone and luteinizing hormone secreted by adenohypophysis, thus gradually increasing follicular development and estrogen secretion.

At this time, estrogen stimulates the endometrium to enter the proliferative phase. Luteinizing hormone increases progesterone secretion, leading to ovulation. During this period, the levels of estrogen and progesterone increased. This has negative feedback inhibition on hypothalamus and pituitary gland, thus reducing the levels of ovulation stimulating hormone and luteinizing hormone, leading to luteal degeneration, and further reducing the levels of estrogen and progesterone.

The endometrium loses the support of these two hormones and falls off and bleeds, which means menstruation. At this time, the decrease of estrogen and progesterone begins the next menstrual cycle.