"There was a linear relationship between gestational age and academic achievement in 3rd grade," explains Dr. Kimberly Noble.
Dr. Noble and colleagues at Columbia University compared data on more than 128,000 children in New York City.
All of the kids were considered full term at birth, at least 37 weeks.
Those born at 37 or 38 weeks scored worse on reading and math achievement tests in the 3rd Grade relative to children born at 39, 40 or 41 weeks.
"Although this is a statistically significant difference, it's a small difference. That's about the size of 1.5 IQ points," Dr. Noble says.
This does not mean an earlier birth causes or leads to academic problems, but the study does suggest the brain is growing very rapidly during those last few weeks in the womb.
"One possibility is that there's something about the intra-uterine environment that really supports optimal brain development," says Dr. Noble.
While more study of this association is needed, experts say it's parents who are most influential in a child's academic development, which can be strengthened with a cognitively stimulating and emotionally supportive childhood.
The effect of an earlier birth on math and reading achievement was independent of birth weight and other obstetric, social and economic factors.