Why does labor begin?

What starts labor is one of the great mysteries of medicine. "That's a Nobel prize-winning question," says Dr. Mark Taslimi, a professor of obstetrics and gynecology at Lucille Packard Children's Hospital in Palo Alto, California. One current theory is that stress hormones are released from the adrenal gland of the baby, which creates chemical changes in the placenta that cause the muscle of the uterus to develop involuntary contractions that turn into labor, he says. These regular, frequent contractions and the resulting opening of the cervix signal that labor has begun.