A study in the Journal of Pediatrics finds that even among those considered full term, between 37 and 41 weeks, being born a week or two later results in more brain development.
Those born on the shorter end of that range scored lower on math and reading tests.
That compares with eight-year-olds born later, but researchers caution the differences were small and should not be alarming.
Some experts say the results suggest the definition of prematurity should be reconsidered.
The findings also raise questions about speeding up childbirth by scheduling elective C-sections.