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 worse than 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.