Heads up, pregnant mamas–if you’re closing in on week 37 of your pregnancy and are excited because you thought your baby was finally going to be considered “full-term,” think again. The American College of Obstetricians and Gynecologists (ACOG) announced yesterday that they are changing the definition of what is considered to be a full-term pregnancy. Formerly, a baby that was born between 37-42 weeks was considered “full-term,” but now babies have to be born no earlier than 39 weeks to be full-term.

“Weeks matter,” said Dr. Jeffrey Ecker of Massachusetts General Hospital, who chaired the ACOG committee that redefined the labels. “Let’s not call it all the same.”

Research shows that babies born before 39 weeks are more susceptible to breathing problems and other issues. A study from 2009 showed that more than one-third of elected C-sections were performed prior to the 39-week mark. Considering that elected C-sections make up nearly a third of deliveries, the reason behind the pregnancy term changes becomes clearer.

According to the National Institutes of Health, studies “pointed toward increased morbidity associated with early-term cesarean section births (37-38 weeks) compared with term neonates (39-41 weeks).” Babies born around 37-38 weeks, which was formerly considered full-term, also needed mechanical ventilation and intubation at higher rates. This all makes sense, as the baby’s lungs are among the last organs to finish developing before he is born.

The March of Dimes and other groups have worked tirelessly to educate women about the risks of delivering a baby prior to the 40-week mark and are thrilled with the changes. Edward McCabe, medical director of the March of Dimes, says that the official endorsement by doctors who deliver babies is “incredibly important.” Otherwise, changing the pregnancy terms won’t do a lot of good.

Check out the new definitions, which were released Tuesday in the journal Obstetrics & Gynecology:

—Early Term, between 37 weeks and 38 weeks 6 days.

—Full Term, between 39 weeks and 40 weeks 6 days.

—Late Term, the 41st week.

—Post Term, after 42 weeks.

While the new definitions provided by the ACOG are intended to decrease the likelihood of any problems that come with being born before the 39-week mark, some people think the more specific definitions could pose problems. Emily Oster, an Assistant Professor of Economics, wrote in her column for Time, “The new guidelines fix one issue, but could lead doctors (in theory) to be too cautious about delivery in 37 or 38 weeks when medically warranted. It could also lead them to be too gung ho about delivery as soon as you hit 39 weeks.”

"Let nature take its course." RT @TIME: New definition of 'full-term' pregnancy presents problems | http://t.co/JA8lNXRHnq

— aubrey quinn (@aubreyquinn) October 23, 2013

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