Even for infants born full-term, a little more time in the womb may matter.
The extra time results in more brain development, and a study suggests perhaps better scores on academic tests, too.
Full-term is generally between 37 weeks and 41 weeks; newborns born before 37 weeks are called premature and are known to face increased chances for 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 - because women are tired of being pregnant or doctors are busy - rather than for medical reasons, the researchers say.
Women should "at least proceed with caution before electing to have an earlier term birth," said 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 and included a sizable number of kids from disadvantaged families. But the authors said similar results likely would be found in other children, too.
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.
"There are still a lot of babies who are being delivered more or less electively at 37 and 38 weeks, with people thinking, `This is no big deal - these babies are full-term.' I think this is a big deal," Aschner said. She was not involved in the study.
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."
Some hospitals including Vanderbilt require obstetricians planning elective C-sections to complete a checklist and if appropriate boxes aren't checked, the operation can't be performed, Aschner said.
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.