New research suggests that birth weight makes a difference to a child's academic performance.

Researchers from Northwestern University in Illinois found that heavier newborns do better in elementary and middle school than infants with lower birth weights.

The study raises an intriguing question: Does a fetus benefit from a longer stay in the mother's womb?

"A child who is born healthy doesn't necessarily have a fully formed brain," David Figlio, one of the study's authors and director of Northwestern's Institute for Policy Research (IPR), said in a statement. "Our study speaks to the idea that longer gestation and accompanying weight gain is good."

Birth weight is a common indicator of a baby's health. Using a major new data source -- merged birth and school records for all children born in Florida from 1992 to 2002 -- the researchers studied the relationships between birth weight and cognitive development by following more than 1.3 million children and nearly 15,000 pairs of twins from birth through middle school.

The findings suggest that babies who weigh more at birth have higher test scores from third through eighth grade. The relationship is apparent even among twins; heavier-born twins have higher average test scores in third through eighth grade than their lighter-born twin.

Even the advantage of attending a higher quality school was not enough to compensate for the disadvantage of a lower birth rate, according to the study. The low birth-rate advantage held up across the board for all children -- regardless of race, socioeconomic status, enrichment experiences provided by parents, maternal education and a host of other factors.

"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," said Figlio, IPR faculty fellow and Orrington Lunt Professor of Education and Social Policy and of Economics at Northwestern's School of Education and Social Policy.

Still, birth weight doesn't seal a child's fate. Children with low birth rates can -- and do -- perform better in school than their heavier peers. Other factors, such as whether a mother graduated from college, can be a larger predictor of academic achievement.

The findings are detailed in the journal American Economic Review.