Doctors think the extra time in the womb results in more brain development.
A new study from the journal "Pediatrics" suggests scheduling your baby's delivery date could affect brain development.
Full-term babies, born between 39 and 41 weeks, mostly did fine on third-grade math and reading tests.
Children who were born at 37 or 38 weeks did slightly poorer than those 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.