The cervix plays a major role in childbirth. As the fetus descends within the uterus in preparation for birth, the presenting part, usually the head, rests on and is supported by the cervix. As labour progresses, the cervix becomes softer and shorter, begins to dilate, and withdraws to face the anterior of the body. The support the cervix provides to the fetal head starts to give way when the uterus begins its contractions. During childbirth, the cervix must dilate to a diameter of more than to accommodate the head of the fetus as it descends from the uterus to the vagina. In becoming wider, the cervix also becomes shorter, a phenomenon known as effacement.