'Full-term' pregnancy gets a new, narrower definition

• The idea that pregnancies are %22at term%22 from 37 to 42 weeks is outdated%2C doctors say
• Babies do best in the two weeks starting at 39 weeks%2C recent research shows
• Doctors hope to discourage elective deliveries before 39 weeks

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 But that is now officially outdated wisdom, two leading medical groups said Tuesday.

A pregnancy is "full term" only in the narrower 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 say.

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, says Jeffrey Ecker, a specialist in maternal-fetal medicine at Massachusetts General Hospital, Boston. He chairs the college's committee on obstetric practice.

The new definitions are based on recent research. The old terminology "was based on the general observation that babies born after 37 weeks tended to do quite well," Ecker says. That's still true, he says, but doctors know now that babies born at 39 and 40 weeks do better and that risks rise again after 41 weeks.

"Language and labels matter," he says, and the changes will help doctors better communicate the latest science to patients.

Here's how expectant 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 0 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

• Post term: 42 weeks and beyond

The definitions were developed at a workshop in 2012 that included representatives from the National Institute of Child Health and Human Development and other medical groups. Details from the workshop were published in the journal JAMA earlier this year.

Still, the official endorsement by doctors who deliver babies is "incredibly important," says 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."

A 2009 survey of 650 women who had recently given birth, also published in Obstetrics & Gynecology, found plenty of confusion about where that goal line was. When asked when a pregnancy reached "full term," 24% said 34-36 weeks, 51% said 37-38 weeks and just 25% said 39-40 weeks.

Those attitudes helped explain why early elective deliveries were continuing to rise even after studies revealed risks for babies, including more breathing and feeding problems and a small, but increased, risk of death.

Groups including the March of Dimes have been working for several years to educate women about those risks. Many hospitals now have programs in place to stop elective early deliveries.

Those efforts have had a "significant effect," but the new terminology will help too, says Elliott Main, medical director of the California Maternal Quality Care Collaborative. "It's really nice for doctors to be able to say this is a national effort and we are trying to improve outcomes. Mothers want better outcomes for their kids."

Still, the cautions on early deliveries "should not panic women who go into labor spontaneously at 37 weeks," or who need to deliver early because they or their babies have medical problems, Ecker says. Early delivery can be justified by conditions from dangerously high blood pressure in a mother to poor growth in a baby, he says. "We are not saying deliveries before 39 weeks should be a 'never' event, but we are saying there should be a conversation."

Main says one reason planned early deliveries have higher risks is that doctors and patients sometimes miscalculate due dates. Babies themselves are better judges, he says, as evidenced by the fact that women who go into spontaneous labor at 37 or 38 weeks tend to have healthier babies than those who choose such early deliveries.

"Spontaneous labor set off by the baby is sign that the baby is really ready to be born," he says.