Labor

"Labor" is the term for the process in which muscular contractions force the fetus through the birth canal. Once labor starts, rhythmic contractions that begin at the top of the uterus and travel down its length force the contents of the uterus toward the cervix. Since the fetus is usually positioned with its head downward, labor contractions force the head against the cervix. This action causes the cervix to stretch, which is thought to elicit a reflex that will stimulate still stronger labor contractions until a maximum effort is achieved. At the same time, the cervix dilates and, as labor continues, abdominal wall muscles are stimulated to contract and aid in forcing the fetus through the cervix and vagina to the outside. Following the birth of the fetus, the "placenta," which remains inside the uterus, becomes separated from the uterine wall and is expelled by uterine contractions through the birth canal. This expulsion is called the "afterbirth."