Defining “Term” Pregnancy

The definition of “term” gestation is often confusing because of the emphasis on reducing preterm birth and the increase in scheduled deliveries. Because preterm is defined as delivery at less than 37 weeks and postterm as more than 42 weeks, “term” has been defined as delivery at 37 to 42 weeks, that is, between 37 weeks 0 days and 41 weeks 6 days. In 2005, deliveries between 34 weeks 0 days and 36 weeks 6 days were changed from near term to late preterm because these infants experience morbidities and mortality similar to those of preterm infants and because of the known outcomes of delivery at less than 37 weeks. The label “term” for pregnancies of 37 to 41 weeks 6 days’ gestation was unchanged. Recent data demonstrate that maternal and neonatal adverse outcome rates are not the same over this 5-week gestational age range. The frequency of adverse outcomes has a nadir at ∼39 to 40 weeks 6 days’ gestation. With advances in determining the conception date, gestational age can be more accurately determined than in years past. Using the same broad gestational age range to define a term pregnancy may be inaccurate. In recent years, data have shown a shift toward earlier gestational age at delivery. Fetal mortality risk is higher at more than 42 weeks compared with 38 to 41 weeks 6 days and at 37 to 38 weeks 6 days than at 39 to 41 weeks 6 days. Infant mortality is lowest at 39 to 41 weeks. Rates for newborn morbidities are lowest at 39 to 40 weeks 6 days, with higher rates at 37 to 38 and more than 41 to 42 weeks. Infants delivered at 37 to 38 weeks 6 days are at higher risk of neonatal morbidity and mortality than those born at 39 to 41 weeks 6 days. The workshop group redefined pregnancy as early term for births between 37 weeks 0 days and 38 weeks 6 days and full term as those between 39 weeks 0 days through 40 weeks 6 days. Deliveries at 41 weeks 0 days through 41 weeks 6 days are considered late term. Given the known excess morbidity and mortality risk of delivering at less than 39 weeks, non–medically indicated delivery before this date should be avoided. Delivery is appropriate for pregnancies between 37 weeks 0 days and 38 weeks 6 days with a medical indication for delivery or spontaneous onset of labor or membrane rupture. An ultrasound examination at 18 to 20 weeks can confirm dating simultaneously with the fetal survey. Women with uncertain menstrual dates should obtain early sonography for optimal dating. This workshop group recommends dividing the 5-week period previously defined as “term” into early term, full term, and late term. More precise language used by clinicians and the public can influence timing of delivery and affect health outcomes for women and children.