Active Phase The cervix opens from about 1 1/2inches (4 centimeters) to the full 4 inches (10 centimeters). It thins and pulls back (effaces) until it merges with the rest of the uterus. The presenting part of the baby, usually the head, begins to descend into the woman’s pelvis. The woman begins to feel the urge to push as the baby descends, but she should resist it. Pushing too early may tear the cervix and wastes energy. This phase averages about 5 to 7 hours in a first pregnancy and 2 to 4 hours in subsequent pregnancies.