Menstruation occurs in about 2 days when the endometrium is 1.2 cm thick. Clinically, the endometrium is thickened before menstruation, and then due to the withdrawal of hormones, the endometrium experiences withdrawal bleeding, which is clinically known as menstruation. However, if there is still no menstruation after 2 days, pregnancy must be ruled out because the endometrium will also thicken after pregnancy under the effect of estrogen. During the menstrual cycle, the endometrium does not remain unchanged. From the time of menstruation, the endometrium slowly moves from the proliferative phase to the ovulation phase, during which the thickness of the endometrium must reach at least 0.9 cm, which is conducive to the fertilization of the egg, and then reaches the secretory phase, during which the endometrium gradually thickens, and when it reaches 1.2 cm, it clinically indicates the onset of menstruation.