The signs of labor can differ from one woman to another, and even for the same woman they may change from one pregnancy to the next. Although there's no way to predict how a woman's labor will progress or how long it will last, there are a few common signs of labor. What initiates labor, however, is less clear.

"We don't really know what mechanisms bring on labor," said Leslie Ludka, a certified nurse midwife and the director of midwifery at Cambridge Health Alliance in Cambridge, Mass. "Science hasn't figured this out yet."

Labor is likely initiated when certain hormones are produced by the fetus, according to the journal Molecular Endocrinology. Below is more information on the physical signs of labor.