Two weeks or so before birth, the fetus "drops"; its head lies against the cervix, which may begin to dilate. When the fetal brain has sufficiently developed, the fetal hypothalamus causes the anterior pituitary to signal the fetal adrenal cortex to release androgens. These male hormones are used by the placenta to produce estrogen, prostaglandins, and oxytocin. All of these trigger the uterine contractions that expel the fetus.
