The cervix of uterus, also known as the cervix or uterine cervix, attaches the vagina to the uterus. It is approximately four centimeters long, approximately half of which extends into the vaginal canal; however, the length of the cervix can be affected by age and childbirth. The uterine cervix produces a mucus that aids in carrying sperm from the vagina to the uterus, where it can fertilize an egg if the woman is ovulating. When the woman isn't ovulating or menstruating, the cervical mucus thickens and serves as a barrier to keep sperm out of the uterus. During childbirth, the cervix thins out and dilates to 10 centimeters to allow the baby to pass through the birth canal. Once the baby is born and the placenta is expelled, the cervix begins to thicken and close. Cancer sometimes develops in the cervix of uterus. Dysplasia is a fully treatable pre-cancer condition of the cervix diagnosed via a pap smear. If dysplasia isn't diagnosed and treated, cervical cancer, which is usually caused by the human papilloma virus, begins to spread.