In healthy women of childbearing age, the onset of menstruation can occur within 5-7 days with endometrium up to 10 mm thick. In patients with endometrial hyperplasia, the thickness of the endometrium does not determine when menstruation occurs. The thickness of the endometrium varies cyclically with the level of hormones secreted by the ovaries, with the endometrium being the thinnest at the end of the menstrual period at about 4 mm, 6-8 mm during ovulation, and 8-12 mm before the onset of menstruation. Under normal circumstances, the ovary forms a corpus luteum after ovulation and secretes progesterone. If no conception occurs, the corpus luteum atrophies, the ovary enters the next ovulation cycle, and the progesterone level drops leading to endometrial exfoliation, then menstruation occurs. If pregnancy or some gynecological endocrine diseases cause endometrial hyperplasia, estrogen and progesterone show high levels, and the endometrium continues to grow, there will be no menstruation. It is not possible to judge the menstrual cycle simply by the thickness of the endometrium. If a thicker endometrium is present in repeated examinations before menstruation, lesions such as endometrial hyperplasia or endometrial polyps should be excluded.