The cascade of hormones that cause labor contractions is begun by the baby himself.

The long, complicated process of labor and deliver is believed to be started by the baby herself. When the baby reaches full size she releases hormones that in turn trigger the release of a number of other hormones by the mother that cause the muscles of the uterus to contract. These same chemicals also slow the breathing movements of the fetal lungs, which carry amniotic fluid in and out, which had become regular earlier in the third trimester. During labor, the movements will cease almost completely, then start suddenly and energetically when the baby enters the world.