Contrary to what you may have heard, as far as we know, labor is not brought on by your favorite Mexican food or a large pizza with the works. In fact, exactly what causes labor to begin is unknown. Today's theories on this topic suggest that there are signals from the baby (I'm ready to come out), from the mother (I'm ready to get rid of this passenger), or from the placenta (I've done my job). Making the matter more complex is the problem of why some women go into labor prematurely (before thirty-seven weeks), and it is likely that whatever triggers preterm labor is different from what triggers full-term labor .