It’s well known that a baby with a low birth weight — below 5.5 pounds — may have health problems. Many of these children are born prematurely, while others have a low weight because of health or substance problems stemming from their mothers. Birth under any of these circumstances can impede a child’s development in the womb, causing respiratory and heart problems, among others. Though its effects on cognitive development are debated, there’s a general consensus that these babies grow up to have difficulty in school. New data supports this idea, and even finds that fatter babies may have the upperhand in their ability to achieve.

“Birth weight matters, and it matters for everyone,” David Figlio, a professor at Northwestern University’s Institute for Policy Research and co-author of the study, told The New York Times.

Figlio and his team are currently working on the study, which compares birth weight of over 75 percent of Florida kids born between 1992 and 2002 to their school records. After accounting for factors like socioeconomic status and parental education, the researchers found an increasing birth weight correlated with an increase in scores on reading and math tests between third and eighth grade. The relationship could be seen as early as kindergarten, and was consistent even among twins, suggesting that genetics didn’t play a part.

Birth weight predicted school grades until about 10 pounds, at which point grades began to fall with increasing weight. After all, going too far to the other side of the birth weight spectrum comes with its own health risks. At such a young age, overly large babies are already at risk of obesity, heart disease, diabetes, and potential neurological problems. The birth weight sweet spot seems to be between 5.5 and 10 pounds.

The researchers concluded that “the effects of poor neonatal health on adult outcomes are largely determined early — in early childhood and the first years of elementary school.” Nevertheless, they say that some of the factors that lead to these deficits may be “remediated,” pointing to their finding that all the kids whose parents were educated performed better in school, “indicating that ‘nurture’ can at least partially overcome ‘nature.’”

Babies may also benefit from being in the womb for longer. Last year, the American College of Obstetricians and Gynecologists and the Society for Maternal-Fetal Medicine published new definitions for full-term pregnancy, narrowing the time frame from between 37 and 42 weeks to 39 weeks and longer. Their reason for doing so: To discourage doctors and patients from performing medically unnecessary deliveries either through induction or C-section, USA Today reported.

While these procedures are safe, and many mothers in need of them benefit, they’ve become too commonplace in the general population. Babies who aren’t ready to be born end up coming out the womb before they’ve finished developing. Researchers have urged doctors and patients alike to allow a child to come out on their own. “Spontaneous labor set off by the baby is a sign that the baby is really ready to be born,” Elliott Main, medical director of the California Maternal Quality Care Collaborative, told USA Today.

Source: Figlio D, Guryan J, Karbownik K, Roth J. The Effects of Poor Neonatal Health on Children's’ Cognitive Development.  American Economic Review. 2014.