Researchers have known that babies born premature are at risk for slowed brain development, but a new study suggests that even among those considered “normal term” - between 37 and 41 weeks - a couple of extra weeks in the womb might make a difference.

Kids born on the shorter end of that range scored lower on math and reading tests as eight-year-olds than those born later - but the differences were small and “shouldn’t be alarming,” one researcher who wasn’t part of the study team said.

“Certainly the vast majority of 37-weekers and 41-weekers would end up developing typically,” said Dr. Kimberly Noble, the lead author on the new study from Columbia University Medical Center and New York-Presbyterian Hospital in New York.

Still, she said, until more research is done, “We would urge caution to both parents and physicians when considering early elective delivery.”

Noble and her colleagues compared birth records and third-grade standardized test scores for 128,000 kids born in New York City in the late 1980s and early 1990s who went to citywide public schools. All of them had been born between 37 and 41 weeks’ gestation, considered normal.

On both reading and math exams, where a score of 50 was considered average, kids born at 41 weeks scored about one point higher, in general, than those born at 37 weeks, the researchers reported Monday in Pediatrics.

That’s equivalent to about a 1.5-point difference on an IQ test, Noble said.

“That would not be a difference that would likely be noticeable from one child to the next,” she told Reuters Health.

“Where it is more noticeable is on the lower end of the (test-score) distribution.”

For example, kids born at 37 weeks were 23 percent more likely to have at least moderate reading impairment, and 19 percent more likely to have moderate math impairment, than those born on the late end of the term range.

Noble said the finding doesn’t prove being born early-term can slow kids’ brain development and hurt their academic achievement. It’s possible, she said, that some other factor is related to both early births and academic difficulties.

Dr. Marie McCormick, a maternal and child health researcher from the Harvard School of Public Health in Boston, agreed that was one limitation of the new study.

“What they really have not been able to control for is any complications that led to the earlier delivery,” said McCormick, who wasn’t involved in the new research.

“Even if it’s an early term delivery, there may have been something going on that led to that child being born earlier in the process than later.”

Still, she told Reuters Health, the findings are consistent with some previous research suggesting babies born at 37 or 38 weeks may be different from those born slightly later.

“It’s been known that at the earlier end at the term range, the kids don’t perhaps do as well as the kids who have the full 40 weeks,” McCormick said.

The researchers agreed that although the findings shouldn’t be too concerning, they are something to consider for women who have some control over when their babies will be born, such as those scheduling a cesarean section.

“The main thing is… when you’re coming to the discussion about delivery and if you have a decision about the timing of that delivery, to really make sure that you’re as far along in pregnancy as you can get without getting out of the range of normal,” according to McCormick.

SOURCE: Pediatrics, online July 2, 2012.