Babies born between 39 and 41 weeks of pregnancy fare better than infants born during weeks 37 or 38, a study shows.

A term birth is considered 37 to 41 weeks, but the new study suggests it is more of a continuum.

The findings appear in Obstetrics & Gynecology.
Previously it was thought that infant mortality risk was similar for babies born between 37 and 41 weeks. But the new study shows that this risk is increased when babies are born between 37 and 39 weeks of gestation, and that this risk decreases with each additional week of pregnancy.

“A truly elective delivery should wait until at least 39 weeks,” says study researcher Uma M. Reddy, MD, MPH of the Eunice Kennedy Shriver National Institute of Child Health and Development in Bethesda, Md.

“We don’t stop labor at 37 weeks, and the majority of babies will do fine,” she says. But “if nothing is going on and you are doing well, waiting to 39 weeks is best for you and your baby.”

Overall risk of infant mortality is low

The overall risk of newborn death remains small, but it doubles for infants born before 37 weeks, compared to infants born at 40 weeks. In 2006, infant mortality was 1.9 deaths for every 1,000 live birth when babies were born at 40 weeks. This rate increased to 3.9 per 1,000 live births when a baby was born at 37 weeks, the study showed.
“Don’t pressure your doctor to deliver you any earlier,” “Reddy says. “The definition of term is arbitrary and 37 weeks isn’t magical because it is the beginning of term.”

Researchers looked at data on 46,329,018 births that occurred from 37 to 41 weeks gestation between 1995 through 2006.
The new study did not include information on the type of delivery (caesarean section vs. natural birth) and/or the reasons for preterm delivery.

Redefining ‘term’

“We need to make people aware that within the definition of term, there are two categories,” says Alan R. Fleischman, MD, the medical director of the March of Dimes in White Plains, N.Y.

Term is “a biologic continuum between 37 and 41 weeks and those babies born early in term are different than those babies born at full term,” he says.

The new findings apply only to women who do not have medical reason to deliver before 39 weeks, he says.

But “there is a whole group of women and doctors who are delivering early for convenience or due to a very minor change in some test that has not been shown to be helpful,” he says. “It is a perfect storm because you never see a mother who likes the last four weeks of pregnancy. It’s uncomfortable and many feel it is time.”

J. Christopher Glantz, MD, MPH, a professor of maternal-fetal medicine at University of Rochester School of Medicine, agrees. “The definition of term is a little arbitrary and it is an old definition based on how babies did many many years ago.”

Most babies do OK at 37 weeks, he says. “The differences are not that dramatic between the actual risk of mortality from 37 to 39 weeks, but why take any unnecessary risks at all?”

“Elective deliveries prior to 39 weeks definitely have some increased morbidity,” says Abdulla Al-Khan, MD, the director and chief of maternal and fetal medicine and surgery at Hackensack University Medical Center in New Jersey Other factors such as type of delivery and reason for early term delivery also affect these risks, he says.