Pregnancy is considered "at term" when gestation has lasted 37 complete weeks (occurring at the transition from the 37th to the 38th week of gestation), but is less than 42 weeks of gestational age (occurring at the transition from the 42nd week to the 43rd week of gestation, or between 259 and 294 days since LMP). "Full term" refers to the gestation having lasted 40 weeks from the first day of the mother's last menstrual period. This is the end of gestation on average. Alternatively expressed, this corresponds to a gestational age of 40 weeks and 0 days, or 280 days, or approximately 9 months, and occurs at the transition from the 40th to the 41st week of gestation. On average, it corresponds to an embryonic age of 38 weeks or 266 days.
Events before completion of 37 weeks (259 days) are considered preterm; from week 42 (294 days) events are considered postterm. When a pregnancy exceeds 42 weeks (294 days), the risk of complications for both the woman and the fetus increases significantly.Therefore, in an otherwise uncomplicated pregnancy, obstetricians usually prefer to induce labour at some stage between 41 and 42 weeks.
Birth before 39 weeks by C section, even if considered "at term", results in an increases risk of complications and premature death, when not medically needed.This is from factors including underdeveloped lungs, infection due to underdeveloped immune system, problems feeding due to underdeveloped brain, and jaundice from underdeveloped liver. Some hospitals in the United States have noted a significant increase in neonatal intensive care unit patients when women schedule deliveries for convenience and are taking steps to reduce induction for non-medical reasons.Complications from Caesarean section are more common than for live births. Recent medical literature prefers the terminology preterm and postterm to premature and postmature. Preterm and postterm are unambiguously defined as above, whereas premature and postmature have historical meaning and relate more to the infant's size and state of development rather than to the stage of pregnancy.