The cervix is the lower portion of the uterus or womb. More specifically, it is the narrow passage that connects the uterus to the vagina. It is the only part of the uterus that is visible, and therefore accessible, to a doctor for sampling tissue as in the case of pap smears.
A tiny opening in the uterus, called the os, remains open when a woman is not pregnant, allowing menstrual blood to flow out of the uterus and sperm to enter it. When a woman becomes pregnant, the os closes tightly to protect the fetus. When a woman is ready to give birth, the os opens again and the cervix begins to widen, or dilate, to allow the fetus to pass out of the uterus and into the birth canal.