Women’s menstrual period is also commonly referred to as menstruation. Menstruation is cyclical, with the cyclical shedding of the uterine lining and bleeding occurring as a result of the cyclical changes in the ovaries, as explained below: The female reproductive organs mainly include the uterus, ovaries, fallopian tubes, and vagina. The ovaries can produce egg cells, thus synthesizing estrogen and progesterone. When the follicle matures and ovulates in the female body, the follicle wall will collapse, similar to the “empty house”, which is the corpus luteum, which mainly secretes progesterone and also synthesizes estrogen. The lining of the uterus changes cyclically with the ovaries. Estrogen can increase the number of endometrial cells, make them larger, and thicken the lining of the uterus, which is known as the proliferative phase of the endometrium. After ovulation, under the joint action of estrogen and progesterone, the endometrium undergoes edema, which is called secretory phase endometrium. About 14 days after ovulation, the corpus luteum begins to shrink gradually and stops secreting estrogen and progesterone. At this time, the blood vessels in the endometrium of the woman will contract, and the endometrium will gradually necrose and fall off, which will cause bleeding and the onset of menstruation.