New definitions of preterm and full term pregnancies have been released by the American College of Obstetricians and Gynecologists.
Until now, a baby was considered preterm if born before 37 weeks of pregnancy and full term if born anytime from 37 to 42 weeks, the Associated Press reported.
The new definitions are: 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. On average, a pregnancy lasts 40 weeks.
The updated classifications were published Tuesday in the journal Obstetrics & Gynecology.
The new definition of a full term pregnancy is meant to reflect the fact that even at the end of the last trimester, a bit more time in the womb can benefit a baby's development and health.
"Weeks matter," Dr. Jeffrey Ecker of Massachusetts General Hospital, chair of the ACOG committee that came up with the more specific labels, told the AP. Since babies' outcomes can differ, "let's not call it all the same," he said.
In recent years, experts have emphasized that that elective deliveries -- inductions and cesarean sections scheduled without a medical reason -- shouldn't be performed before the 39th week of pregnancy. Studies show that infants born at 37 weeks have a higher risk of complications, such as difficulty breathing, than those born just two weeks later.
The new definitions were welcomed by the March of Dimes, which said they will eliminate "confusion about how long an uncomplicated, healthy pregnancy should last," the AP reported.