The thickness of the endometrium varies with the menstrual cycle. For example, when the period is just cleared, the endometrium is thin. With the increase of estrogen until the increase of progesterone before menstruation, the endometrium can be thickened, which is a physiological change of the endometrium, which occurs under the action of ovarian hormones. However, there are some pathological conditions that can also thicken the endometrium. For example, clinically, functional uterine bleeding can cause abnormal thickening of the endometrium, even simple hyperplasia or complex hyperplasia. Also if the endometrium is thickened before pregnancy, this facilitates the fertilization of the egg.