"We have increasingly recognized that newborn outcomes are not uniform between 37 and 42 weeks," Dr. Jeffrey Ecker said.
Babies delivered between 37 weeks and 39 weeks of pregnancy will now be considered "early term," according to the American College of Obstetricians and Gynecologists.
"Full term" infants will be those born between 39 and 41 weeks.
Babies born between 41 and 42 weeks of pregnancy will be thought of as "late term." Finally, those born at 42 weeks or later will still be considered "postterm."
Ecker is the chair of The College's Committee on Obstetric Practice. He is also a high-risk obstetrician at Massachusetts General Hospital in Boston.
"Language is important in communicating that it's not just one period of time and to recognize that outcomes do differ," he told Reuters Health.
A growing body of research has found babies born before 39 weeks are not as developed as those born later.
Babies born after 39 weeks have fewer poor outcomes such as breathing, hearing and learning problems, The College says in its joint statement with the Society for Maternal-Fetal Medicine. The statement was published Tuesday in Obstetrics and Gynecology.
The brain grows by about a third between week 35 and week 39 of pregnancy, according to The College. And a layer of fat to help keep the body warm is added during the last weeks of pregnancy.
Image: Pregnant woman's belly, via Shutterstock