Defining “Term” Pregnancy

Abstract

THE NATIONAL EMPHASIS ON REDUCING PRETERM BIRTH, and the increase in scheduled deliveries, has created confusion around the definition of term gestation. The concept of “term” gestation provides guidance to clinicians and influences the public’s perceptions about the optimal timing of delivery for a healthy pregnancy. Currently, a term birth is defined as a delivery that occurs neither preterm nor postterm. Because preterm is classically defined as delivery prior to 37 weeks following the onset of the last menstrual period and postterm as after 42 weeks, term has been conventionally defined as delivery between 37 and 42 weeks. The International Classification of Diseases defines term pregnancy as delivery between 37 weeks 0 days and 41 weeks 6 days. In 2005, a US National Institute of Child Health and Human Development workshop changed the designation of deliveries between 34 weeks 0 days and 36 weeks 6 days from near term to late preterm to emphasize that these infants experience morbidities and mortality similar to those of preterm infants and to highlight the adverse consequences of delivering before 37 weeks. This nomenclature acknowledged that fetal maturation is a continuum, yet the use of the label “term” for pregnancies spanning 37 weeks 0 days through 41 weeks 6 days’ gestation remained unchanged. Recent data demonstrate that maternal and neonatal adverse outcome rates are not the same across the 6-week gestational age range that constitutes term. Rather, the frequency of adverse outcomes is U-shaped, with the nadir around 39 weeks 0 days through 40 weeks 6 days’ gestation. Before the advent of more accurate methods, last menstrual period was used for pregnancy dating because women usually remembered that date but not the date of conception. With the advent of ultrasound, accurate early home pregnancy tests, assisted reproductive technologies, and home ovulation test kits, the actual timing of conception, and consequently of gestational age, is more accurately determined than in the past. Therefore, it may not be as appropriate to continue using the same broad gestational age definition of term as when dating was less accurate. To refine further the definition of term, the National Institute of Child Health and Human Development, the American Congress of Obstetricians and Gynecologists (formerly the American College of Obstetricians and Gynecologists), the American Academy of Pediatrics, the Society for Maternal-Fetal Medicine, the March of Dimes, and the World Health Organization (WHO) convened a meeting of experts and stakeholders on December 17, 2012, in Bethesda, Maryland, and this article reports recommendations from that workshop.