Preschoolers whose mothers regularly ate low-mercury fish during pregnancy may have sharper minds than their peers, a study suggests.

Researchers found that among 341 3-year-olds, those whose mothers ate more than two servings of fish per week during pregnancy generally performed better on tests of verbal, visual and motor development.

On the other hand, tests scores were lower among preschoolers whose mothers had relatively high mercury levels in their blood during pregnancy.

And mothers who regularly ate fish during pregnancy were more likely to have such mercury levels than non-fish-eaters were, the researchers report in the American Journal of Epidemiology.

The findings add to evidence that fish can be brain-food, but underscore the importance of choosing lower-mercury fish during pregnancy.

"Recommendations for fish consumption during pregnancy should take into account the nutritional benefits of fish as well as the potential harms from mercury exposure," write the researchers, led by Dr Emily Oken of Harvard Medical School in Boston.

Oily fish such as tuna, salmon and sardines contain omega-3 fatty acids, which are important in fetal and child brain development. The problem is that fatty fish are more likely to be contaminated with mercury, a metal that is toxic to brain cells, particularly in fetuses and young children.