The home for a developing fetus is the amniotic sac. Entering the birth canal requires leaving the sac. The sac membrane breaks and the amniotic fluid that filled it – about a quart by the final week of pregnancy – flows out of the vagina.
The amniotic sac membrane is thought to help protect the baby’s head during labor, against the opening cervix. Sometimes the sac membrane breaks naturally; when a woman’s so-called “water breaks,” and sometimes the sac is ruptured by the birth attendant.