The onset of labor, leading to delivery, before at least 37 weeks have elapsed since the last menstrual period. The more prematurely babies are born, the greater the difficulty they have surviving, even with expert care.
labor beginning prior to the 37th week of gestation
regular contractions before 38 weeks of pregnancy.
Also preterm labor; labor occurring before 37 full weeks of pregnancy.
Labor that occurs after 20 weeks gestation but before 37 weeks.