The normal uterine cavity is 7-8 cm deep, but there are individual differences. The uterus is the organ that nurtures the fetus and produces menstruation. The normal uterus of a woman of childbearing age is inverted pear shaped, with a width of 4-5cm, a depth of 7-8cm, and a volume of about 5 ml. The uterus is divided into the body of the uterus and the cervix, with the body of the uterus being wider and situated in the upper part of the uterus, and the uterine horns being located at the base of the uterus on both sides. After pregnancy the uterus gradually enlarges to accommodate the pregnancy load. The lining of the uterus is periodically affected by the sex hormones secreted by the ovaries and undergoes cyclic shedding to form menstruation. The formation of regular menstruation is a criterion for the maturity of female reproductive function. The narrowest part of the uterus between the uterine body and the cervix is the isthmus of the uterus, which at the end of pregnancy forms the lower part of the uterus, part of what is called the soft birth canal, and is a common site of incision for cesarean sections.