The lining of the uterus is made up of the endometrium, and along with the menstrual cycle, estrogen levels can be significantly higher or lower, which can be a problem for women if abnormalities occur. The endometrium is gradually shed from day 1 to day 4 of the menstrual cycle, so even if menstruation occurs, the endometrium will grow again after the end of menstruation, and the thickness of the endometrium will not exceed 12 mm until the next menstruation, if it exceeds 12 mm, it is endometrial hyperplasia. If it is less than 5 mm, it is a weak endometrium, which is mainly seen in premature ovarian failure or in menopause. From day 5 to day 14 of the menstrual cycle, the epithelium and blood vessels of the endometrium change significantly, and at this stage the endometrium grows from 0.5 mm to about 3 mm, while from day 15 to day 28 of the menstrual cycle, the endometrium gradually thickens and is rich in nutrients, which can effectively promote fertilization of the egg up to a thickness of 10 mm. The normal endometrium is about 10 mm thick before the onset of menstruation and usually does not exceed 12 mm. If it exceeds 12 mm and is accompanied by uneven echogenicity of the endometrium, be alert to the possibility of endometrial hyperplasia and cancer.