The regulatory factor in the childbirth is the pressure on the cervix of the woman. When a pregnant woman undergoes labor, her cervix changes allowing her body to deliver the baby safely. The cervix changes a lot during the labor and the delivery process. It moves to the anterior position from its previous posterior position. The cervix flex along with the uterine contraction and because of the intrauterine pressure. The cervix become effaced during the process of the labor. The ripening of the cervix starts at the end of the pregnancy. It softens and thins and then, dilates during the delivery.
hence, the answer is 'pressure on the cervix'.