The uterus is a hollow, pear-shaped organ in the lower abdomen. It is where a fetus grows during pregnancy. About once a month the lining of the uterus thickens. If a pregnancy doesn't occur, the uterus sheds its lining as a menstrual period.

The lower part of the uterus (cervix) opens into the vagina. The opening to the cervix is narrow except during labor and birth, when it stretches and widens to allow the baby's passage out of the uterus.