Uterus, YOO tuhr uhs, or womb, is a hollow, muscular organ of the female reproductive system in which an unborn baby develops. The organ is near the base of the abdomen. In a woman who is not pregnant, the uterus resembles an upside-down pear in shape and is about the size of a fist. The bottom part of the uterus, called the cervix, contains a necklike opening that leads into the vagina. At birth, a baby passes down through the cervix and the vagina and then out of the woman's body.