In most pregnancies, labor starts between 38 and 42 weeks after the last menstrual period. Labor is considered preterm labor when it starts before the beginning of the 37th week.
Labor starts with regular contractions of the uterus.
It is not known exactly what causes labor to start. Hormones produced by both the woman and fetus play a role. Changes in the uterus, which may be caused by these hormones, may cause labor to start. Preterm labor may be a normal process that starts early for some reason. Or, it may be started by some other problem, like infection of the uterus or amniotic fluid .