(Agencies)
Updated: 2008-04-02 08:04

New York -- 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.

Because of this, pregnant women are advised to avoid certain fish altogether: shark, swordfish, king mackerel and tilefish. These fish are particularly high in mercury because they eat other fish and are long-lived, over time accumulating mercury in their fat tissue.

Less clear is how the benefits of other omega-3-containing fish stack up against the potential risks. Currently, US health officials recommend that pregnant women eat no more than 12 ounces, or roughly two servings, of fish per week.

For the current study, Oken's team collected blood samples from 341 women during their second trimester and asked them how often they ate various foods, including fish. When their children were 3 years old, they took standard tests of vocabulary, visual-spatial skills and fine-motor coordination of the hands and fingers.

Overall, the researchers found, children whose mothers ate fish more than twice a week had higher test scores.

However, children whose mothers had mercury levels in the top 10 percent of the study scored more poorly than those whose mothers had lower mercury levels.

Only 2 percent of mothers who never ate fish during pregnancy had blood mercury levels that high, versus 23 percent of those who ate fish more than twice weekly.

According to Oken's team, the bottom line is that eating fish lower in mercury could "allow for stronger benefits of fish intake."

Fish that are high in omega-3 but relatively lower in mercury include canned light tuna, which has less mercury than albacore tuna, and smaller oily fish like salmon. White-meat fish such as cod and haddock tend to be low in mercury, but have less omega-3 than fattier fish.

The study was funded by the National Institutes of Health and Harvard. Some of Oken's co-researchers have received funding from the food and supplement industry.