Babies who weigh more at birth perform better in elementary and middle school than infants with lower birth weights, suggests research published in the Dec. 14 issue of the journal American Economic Review.

The study, led by researchers at Northwestern University, explores whether a fetus may benefit from a longer stay in the mother’s womb.

“A child who is born healthy doesn’t necessarily have a fully formed brain,” study author David Figlio, director of Northwestern University’s Institute for Policy Research, said in a news release. “Our study speaks to the idea that longer gestation and accompanying weight gain is good. We want to know: What does that mean for public policy?”

According to the news release, the research is the first to analyze the association of school quality and the relationship between birth weight and children’s cognitive development.

“The results strongly point to the notion that the effects of poor neonatal health on adult outcomes are largely determined early— in early childhood and the first years of elementary school,” the researchers wrote in the study.  

The researchers studied merged birth and school records for all children born in Florida from 1992 to 2002. They analyzed the relationships between birth weight and cognitive development in more than 1.3 million children and about 15,000 pairs of twins from birth through middle school.

Babies who weighed more at birth had higher test scores from third through eighth grade, the study authors found. The trend held true for twins, too: For any given pair of twins, the heavier-born sibling had a higher average test score during these school years than his or her lighter-born sibling.

Being born at a heavier weight proved advantageous regardless of other factors such as race, socioeconomic status, enrichment experiences provided by parents and maternal education, according to the news release.

“It will be valuable to learn whether improvements in earnings by families with pregnant women, improved maternal nutrition or reduced maternal stress— all factors associated with higher birth weight— also translate to better cognitive outcomes in childhood,” Figlio said in the news release.

Despite these statistics, infants born at a lighter weights still can— and do— excel academically, said study coauthor Jonathan Guryan, associate professor of human development and social policy at Northwestern University.

The researchers pointed out that other factors— like whether the child’s mother graduated from college— can be a larger predictor of academic achievement than birth weight.

“You’d rather be a low birth-weight baby with a mother who has a college degree than a heavier baby born to a high school dropout,” Guryan said.