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] followed 6,250 children from kindergarten through fifth grade and found that those who were obese throughout that period scored lower on math tests than non-obese children.

What’s more, this pattern held even after the researchers took into account extenuating factors that can influence both body size and test scores, such as family income, race, the mother’s education level and job status, and both parents’ expectations for the child’s performance in school.

“In boys and girls alike who entered kindergarten with weight problems, we saw these differences in math performances emerge at first grade, and the poor performance persisted through fifth grade,” says lead researcher Sara Gable, Ph.D., an associate professor of nutrition and exercise physiology at the University of Missouri-Columbia.