Even for infants born full-term, a little more time in the womb may mean better brain development — and academic tests, too.
Full-term is generally between 37 weeks and 41 weeks; babies born before 37 weeks are called premature and are known to face increased risks of health and developmental problems.
The children 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.
The researchers and other experts said the results suggest that the definition of prematurity should be reconsidered.
The findings also raise questions about hastening childbirth by scheduling cesarean deliveries for convenience rather than for medical reasons.
Women should "at least proceed with caution before electing to have an earlier-term birth," said the lead author, Dr. Kimberly Noble, an assistant pediatrics professor at Columbia University Medical Center.
The study involved 128,000 New York City public school children. Of the children born at 37 weeks, 2.3 percent had severely poor reading skills and 1.1 percent had at least moderate problems in math. That compares to 1.8 percent and 0.9 percent for the children born at 41 weeks.
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.
The research "will cause quite a stir," said Dr. Judy Aschner, a pediatrics professor and neonatology director at Vanderbilt University Medical Center.
Aschner said no one is recommending trying to delay childbirth for women who go into labor at 37 weeks or 38 weeks.
"I don't want to panic moms whose babies come at 37 weeks," she said. "But those elective early deliveries really need to stop."