Generally, the thickness of the endometrium is less than 10 mm, but from puberty until menopause, the thickness of the endometrium changes continuously with the menstrual cycle, which is a continuous process of change. During the female physiological cycle, the endometrium is affected by the secretion of female hormones by the ovaries as they undergo a series of cyclic changes. When there are follicles developing and maturing in the ovary, under the action of estrogen secreted by the ovary, the endometrial epithelium and mesenchymal cells show a proliferative state called the proliferative phase, the thickness of the endometrium is 1-3mm, which is in the 5th-14th days of the menstrual cycle, after ovulation, the ovarian corpus luteum is formed, and under the synergistic action of progesterone and estrogen, the endometrium shows a secretory response, which is called the secretory phase, this phase is in the 15-28 days, the thickness of endometrium is 5-10mm, if the egg cell is not fertilized, the ovarian corpus luteum degenerates, the level of estrogen and progesterone decreases, the endometrium loses the support of sex hormones, degeneration, necrosis and exfoliation of tissues occur, the necrotic endometrium is mixed with blood and discharged, at this time menstruation comes, only the basal layer of endometrium is retained in the menstrual period. Therefore, the standard of endometrial thickness examination is different when performed at different stages.