Shortly after fertilization the hormone progesterone increases and is maintained at high levels in the mother's bloodstream. The high levels of progesterone prevent the uterus from contracting. The progesterone prepares the
lining of the uterus (the endonestrium) for its supporting role in nurturing the developing fetus, and helps form the placenta. Maternal progesterone levels begin to drop during the last weeks of gestation, while the levels of estrogen begin to rise. When progesterone levels drop to very low levels and estrogen levels are high, the uterus begins to contract.
Science EncyclopediaScience & Philosophy: Bilateral symmetry to Boolean algebraBirth - Viviparous Animals, Maternal Progesterone, Oxytocin, History Of Childbirth, Types Of Childbirth Preparation, Types Of Anesthesia - How does birth begin?, Fetal endocrine control, Birth in humans