The researchers did try to account for other factors, such as family income and parents' education. But they lacked some key information -- including parents' IQ scores, said Van Lieshout, who has studied the relationship between mothers' weight and kids' development.
"You can't conclude from this that women should try to attain a healthy weight before pregnancy in order to improve their child's cognition," Van Lieshout said.
A child behavior specialist agreed. One question is what mothers' nutrition was like during pregnancy, said Andrea Vazzana, clinical assistant professor of child and adolescent psychiatry at NYU Langone Medical Center in New York City.
Since nutrition during pregnancy affects fetal development, it's possible that differences in diet -- rather than weight, per se -- have some role in the findings.
But there were also problems in how the study assessed kids' abilities, Vazzana said. Their verbal, reasoning and numbers skills were each gauged with one test, or "scale." And that's not enough to broadly capture children's abilities, she said.
There also was a lot of missing data, she pointed out. For instance, the researchers had full information for only about half of the children at age 7.
Still, Van Lieshout said that nine studies so far have looked at the relationship between mothers' weight and children's mental and behavioral development, and most suggest a link.
The question, though, is why. Van Lieshout said the "most tantalizing" theory is that there could be something about mothers' excess fat that affects fetal brain development. Animal research has suggested that's possible, but no one knows if that's true in humans.
For now, the experts said, there are plenty of reasons for women to go in
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