Birth

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 A Holstein calf being born. Unlike a newborn human child it will walk almost immediately. Photograph by Cynthia Matthews. Stock Market. Reproduced by permission.

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.