Labor and delivery are the final phases of a pregnancy, which end with the birth of the baby. Labor begins with physiological changes in the mother’s body that indicate the imminent birth of the child. These changes include:

• Dilation (widening) and effacement (thinning) of the cervix
  
  
• Rhythmic contractions of the uterus that become more frequent and intense as delivery nears

Once a woman has gone into labor, a prolonged, three-stage process begins in which the baby moves down into the pelvis in preparation for birth. Labor often lasts between 12 and 14 hours – or longer – for first-time mothers, but is usually shorter in subsequent births.

In most cases, labor and delivery occur between the 37^th and 42^nd weeks of a pregnancy. The typical U.S. pregnancy is about 39 weeks, according to new analysis by the March of Dimes. In other situations, labor may begin before the 37^th week of pregnancy, a condition known as preterm labor that is potentially dangerous to the health of the fetus. In still other cases, natural labor may be delayed or is not strong enough to produce adequate contractions. In such instances, a physician may need to induce or augment labor to help the birth process advance.