Menstruation is caused by the shedding of the endometrium. The endometrium is gradually changed by the influence of estrogen and progesterone in the body. When menstruation just clears, the endometrium is very thin, at this time the follicles start to develop and the endometrium gradually thickens due to the influence of estrogen secreted by the follicles. After ovulation, the endometrium should reach its thickest state. After ovulation, progesterone is secreted, and estrogen and progesterone act on the endometrium at the same time to enrich the blood flow of the endometrium. If fertilization is possible, it can facilitate the fertilized egg’s implantation. If pregnancy occurs, the endometrium does not shed, but continues to thicken and can supply nutrients to the fertilized egg. If there is no pregnancy, the estrogen and progesterone in the body will drop and the endometrium will shed and bleed because there is no hormonal support.