The normal uterus is pear-shaped, with a volume of about 5ml and a weight of about 50g. The uterus is the place where the fetus is conceived. After pregnancy, the uterus can be extremely enlarged, and the volume can reach 5000ml, which is 1000 times larger than that in the non-pregnant state. The uterus is divided into three layers, the innermost is the endometrium, the lower is the myometrium of the endometrium, and the outermost is the serosa. The endometrium changes with the menstrual cycle. It reaches the thickest before menstruation, falls off during menstruation, and is the thinnest at the end of menstruation, and then grows gradually under the action of estrogen secreted by the ovary. At this time, it becomes the proliferative phase of endometrium, and after ovulation, the endometrium enters the secretory phase to prepare for pregnancy.