To understand how a woman’s menstrual blood comes in, it is important to first understand the cyclical changes in the ovaries as well as the cyclical changes in the endometrial lining of the uterus. After the previous menstrual period the endometrium is at its thinnest state, new follicles develop in the ovaries, and during ovulation the endometrium is in a constant state of hyperplasia due to the constant increase in estrogen levels. At the time of ovulation estrogen levels are high, and at this time the endometrium is in the state of fullest hyperplasia. After ovulation, the level of progesterone will gradually increase, and the endometrium will change from the proliferative phase to the secretory phase, which is due to the effect of progesterone that makes the endometrium appear in the secretory phase. Progesterone is at its peak 7-8 days after ovulation, so the secretory phase of the endometrium is also at its fullest at this time. Thereafter, the level of estrogen and progesterone will gradually decrease due to the lack of fertilization, and when it reaches the lowest peak, the secretory phase of the endometrium will lose the support of the corresponding hormones, and exfoliative bleeding will occur, and menstruation will come. Therefore, women’s menstrual bleeding is due to the low level of estrogen and progesterone, the endometrium loses its supportive effect and exfoliative bleeding occurs.