Children born in the 41st week of pregnancy - which is considered "late-term" - have better test scores and are more likely to be classified as gifted in elementary and middle school, compared with children born "full-term," that is, at 39 or 40 weeks.
"It has been well-established that late-term births are associated with higher levels of neonatal health problems," Dr. David N. Figlio from Northwestern University, Evanston, Illinois told Reuters Health by email. "But due to data limitations, we haven't been able to look well beyond birth to see whether these problems persist, or if there are other potential benefits of late-term births."
The new study, he said, "shows not only that children born in late term continue to have an elevated risk of health problems when school-aged, but also that they have an elevated rate of cognitive benefits."
Dr. Figlio's team compared test scores at ages 8 through 15 on the Florida Comprehensive Assessment Test (FCAT) for more than 320,000 children born early-term, nearly 720,000 born at full-term, and almost 120,000 born late-term. They also looked at whether the children were classified as gifted by the Florida Department of Education.
All of the kids were born between 1998 and 2013.
Late-term infants fared better across the board, with higher standardized test scores, a greater percentage classified as gifted, and a smaller percentage having poor cognitive outcomes, according to a report in JAMA Pediatrics.
On the other hand, late-term infants were also more likely than full-term infants to have abnormal physical conditions of birth and physical disabilities at school-age.
The academic differences between late-term and full-term infants were about half the size of the differences seen between full-term and early-term infants, while the physical disability differences between late-term and full-term infants were about one-third the differences seen between full-term and early-term infants.
The differences between late-term and full-term infants were more pronounced for children of less-educated mothers.
"A statistical study is just one piece of evidence that expectant parents should consider when thinking of the 'right' time to give birth," Dr. Figlio said. "Physicians have a tremendous amount of information about maternal and fetal health in each specific pregnancy."
When a pregnancy is routine, low-risk, and uncomplicated, "there seems to be no 'magic time' to deliver your baby," he said. "There are tradeoffs associated with being born at 39 or 40 weeks versus being born at 41 weeks, and expectant parents should weigh the apparent cognitive benefits against the apparent physical risks when thinking about their most-preferred time to deliver."
"I imagine that some families, in cases of routine, low-risk, uncomplicated pregnancies, would opt for assuming a modestly higher risk of physical issues in order to achieve modestly higher chances of better cognitive outcomes, and other families would opt for modestly lower chances of good cognitive outcomes in order to achieve modestly better chances of physical health," Dr. Figlio said.