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.
Each month during a woman’s childbearing years, blood vessels, glands, and cells build up in the lining of the uterus. This process prepares this organ to receive a fertilized egg. If fertilization does not occur, the built-up lining is discharged during menstruation. If fertilization takes place, the fertilized egg attaches itself to the lining of the uterus. Then the egg develops into an embryo, and tissues from the uterus and the embryo form a disk-shaped organ called the placenta. The placenta provides the unborn baby with food and oxygen and carries away its waste products (see Placenta ).
The uterus expands to about 24 times its normal size during pregnancy, mainly because of an increase in the size of muscle cells in the wall of the organ. During childbirth, the muscles of the uterus contract and force the baby out of the mother’s body. A second wave of contractions expels the placenta. The uterus never completely returns to its prepregnant size, and the opening of the cervix remains slightly changed in shape.
