New definition of ‘full term’ narrows on-time arrival window


To the chagrin of pregnant women terrified of giving birth on a dingy Metro platform, due dates are far from certain. Due dates are also weird: They are calculated as 280 days (or 40 weeks) from the day of a woman’s last menstrual period, rendering a woman officially “pregnant” before she’s even ovulated. The odds of a little bundle of joy arriving precisely on his or her due date are actually pretty low. But thankfully, doctors are pretty good at ballparking the big day.

Until now, babies born at any time during a wide five-week window (three weeks before a due date and two weeks after) were considered fully cooked. Now, a panel of clinicians says otherwise.

In an October 22 announcement and an opinion piece in the November Obstetrics & Gynecology, the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine narrowed the definition of “full term” by shaving off two weeks at the beginning and one at the end. Instead of a luxurious five-week window, babies now have a two-week window to hit. Under the new definition, babies born during weeks 37 and 38 of pregnancy are “early term,” babies who hit weeks 39 and 40 are “full term,” those who arrive during week 41 are “late term,” and babies born beyond week 42 are “post term.”

These narrowed definitions better reflect outcome realities for babies, the scientists write. The old definition assumed that all babies born during that five-week window were equally healthy. But a growing mountain of data says otherwise. Even though they were considered full term, babies born on the early side, around 37 weeks, are known to be at a higher risk for poorer outcomes.

The new definitions are an effort to discourage clinicians and pregnant women from scheduling elective births, either by Cesarean section or by induced labor, before 39 weeks. Even during the last few weeks of pregnancy, the growing body and brain are still cramming in loads of last-minute work, and that important time shouldn’t be cut short for trivial reasons.

Laura Sanders

Laura Sanders is the neuroscience writer. She holds a Ph.D. in molecular biology from the University of Southern California.

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