Multiple hormones play an integral role in labor and birth. When the baby is ready to be born, it signals the mother's body by releasing a series of hormones. During pregnancy, there are high levels of the hormone progesterone circulating in the mother's system. This chemical prevents uterine contractions. Therefore, the first step in the labor process involves a cessation of the release of progesterone. As the fetus grows to the limits of the uterus, stress hormones, particularly corticotropin-releasing hormone (CRH) and cortisol, are released by both the fetus and the mother in the last few days of pregnancy. These stress hormones cause a release of estriol, a particular type of estrogen. Estriol inhibits the release of progesterone. This in turn allows the uterus to contract. As uterine contractions begin, prostaglandin hormones kick in and further prevent the release of progesterone and signal the body to go into labor.
At this point the hormone relaxin causes the cervix to relax, allowing it to stretch wide enough for the baby to leave the uterus. Relaxin, combined with some oxytocin, also allows the mother's hip and pelvic muscles to relax enough to allow the passage of the baby.
After the birth of the baby, oxytocin is released by the mother. This hormone is important for bonding between mother and child. The baby also produces oxytocin at this stage. In the mother, this hormone also signals that labor has come to an end. Throughout this whole process, adrenaline and endorphins are released to help the mother deal with the pain and anxiety of the whole process.