Definition of Term Pregnancy

Gestation in singleton pregnancies lasts an average of 40 weeks (280 days) from the first day of the last menstrual period to the estimated date of delivery. In the past, the period from 3 weeks before until 2 weeks after the estimated date of delivery was considered “term” 1, with the expectation that neonatal outcomes from deliveries in this interval were uniform and good. Increasingly, however, research has identified that neonatal outcomes, especially respiratory morbidity, vary depending on the timing of delivery even within this 5-week gestational age range. The frequency of adverse neonatal outcomes is lowest among uncomplicated pregnancies delivered between 39 0/7 weeks of gestation and 40 6/7 weeks of gestation 2, 3. For this reason, quality improvement projects have focused, for example, on eliminating nonmedically indicated deliveries at less than 39 0/7 weeks of gestation 4.

In order to facilitate data reporting, delivery of quality health care, and clinical research, it is important that all clinicians, researchers, and public health officials use both uniform labels when describing deliveries in this period and a uniform approach to determining gestational age. To address the lack of uniformity in neonatal outcomes between 37 0/7 weeks of gestation and 42 0/7 weeks of gestation, a work group was convened in late 2012 to determine whether term pregnancy should be redefined 5. The work group included representatives from the Eunice Kennedy Shriver National Institute of Child Health and Human Development, the American College of Obstetricians and Gynecologists (the College), the Society for Maternal-Fetal Medicine (SMFM), and other professional societies and stakeholder organizations. The work group recommended that the label “term” be replaced by the designations early term, full term, late term, and postterm to more accurately describe deliveries occurring at or beyond 37 0/7 weeks of gestation Box 1. The group recommended that the use of the label “term” to describe all deliveries between 37 0/7 weeks of gestation and 41 6/7 weeks of gestation should be discouraged. Details of the evidence and rationale that are the foundation of these recommendations can be found in published summaries of this conference 5.

The College and SMFM endorse and encourage the uniform use of the work group’s recommended new gestational age designations by all clinicians, researchers, and public health officials to facilitate data reporting, delivery of quality health care, and clinical research.

Uniform definitions of term are predicated on a uniform method of determining gestational age. The work group provided a method for determination of gestational age 5 that, like other similar methods 6, focused on a hierarchy of clinical and ultrasonographic criteria. Individual methods may differ in the details of when and how ultrasonographic biometry should be used to change estimated date of delivery based on last menstrual period; however, it is not the purpose of this document to establish the priority of one method over another. The College and SMFM are working with other expert groups to establish evidence-based consensus on criteria for determining gestational age.