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Doctors redefine 'full term' pregnancy

| Wednesday, Oct. 23, 2013, 5:42 p.m.

Generations of mothers-to-be have heard that babies born any time between 37 and 42 weeks of pregnancy were “at term” — neither too early nor too late. But that is now officially outdated wisdom, two leading medical groups said.

A pregnancy is “full term” only in the two-week window that starts at 39 weeks, under new definitions published in the journal Obstetrics & Gynecology and endorsed by the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine. The groups say babies born within those two weeks do best. Babies born two weeks before or one week after that window, at “early term” or “late term,” face a few more risks, they said.

The biggest reason the terminology needs to change is to discourage doctors and patients from scheduling medically unnecessary deliveries — by induction or C-section — before 39 weeks, said Jeffrey Ecker, a maternal-fetal medicine specialist at Massachusetts General Hospital, Boston.

The old terminology “was based on the general observation that babies born after 37 weeks tended to do quite well,” Ecker said.

That's still true, but doctors know now that babies born at 39 and 40 weeks do better; risks rise again after 41 weeks.

“Language and labels matter,” he added.

Here's how mothers-to-be mothers should now expect doctors to describe the last possible weeks of pregnancy (counted from the first day of a woman's last menstrual period, but sometimes adjusted after an ultrasound):

• Early term: Between 37 weeks, zero days and 38 weeks, six days.

• Full term: Between 39 weeks and 40 weeks, six days.

• Late term: Between 41 weeks and 41 weeks, six days.

• Postterm: 42 weeks or more.

The definitions were developed last year and published this year in the journal JAMA.

Still, the official endorsement by OB-GYN doctors is “incredibly important,” said Edward McCabe, medical director of the March of Dimes. “In the past, when a woman made it past the 37-week goal line, she was home. This moves the goal line.”

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