updated 7/2/2012 9:27:42 AM ET
msnbc.com news services
Even for infants born full-term, a little more time in the womb may matter.
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 later in school test scores.
The 8 year-old children tracked in the study were all full-term, and the vast majority did fine on third-grade math and reading tests. The differences were small, but the study found that more kids born at 37 or 38 weeks did poorly than did kids born even a week or two later.
"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.
Women should "at least proceed with caution before electing to have an earlier term birth," said Noble, an assistant pediatrics professor at Columbia University Medical Center.
The study involved 128,000 New York City public school children and included a sizable number of kids from disadvantaged families. But the authors said similar results likely would be found in other children, too.
The researchers and other experts said the results suggest that the definition of prematurity should be reconsidered.
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."
Children born at 38 weeks faced only slightly lower risks than those born at 37 weeks.
Compared with 41-weekers, children born at 37 weeks faced a 33 percent increased chance of having severe reading difficulty in third grade, and a 19 percent greater chance of having moderate problems in math.
"These outcomes are critical and predict future academic achievement," said Naomi Breslau, a Michigan State University professor and sociologist. Her own research has linked lower IQs in 6-year-olds born weighing the same as the average birth weights at 37 and 38 weeks' gestation, compared with those born heavier.
The study was published online Monday in Pediatrics.
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.
In the study, 15 percent of children were born in C-section operations but there was no information on how many of these were elective or medically necessary procedures. C-sections can cause birth complications that also increase chances for developmental delays. But the researchers took that into account, along with other risk factors including low birth weight, lack of prenatal care, smoking during pregnancy and neighborhood poverty - all of which could contribute to academic difficulties. And they still found that birth at 37 weeks and 38 weeks was an additional risk.
"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.