Fish can contain mercury, and women with high levels were more likely to have children with ADHD symptoms.
But the same study also finds eating at least two servings of fish a week was linked to about a 60 percent lower risk of developing ADHD-like symptoms.
It may seem contradictory, but one possibility is the nutritional benefits of fish offset the harmful effects of mercury.
Researchers say it's best for pregnant women to avoid big fishes, such as tuna and swordfish, which typically contain the most mercury.
There is more in the Archives of Pediatrics and Adolescent Medicine.