U.S. researchers studied 341 3-year-olds and discovered those whose mother ate more than two servings of low-mercury fish a week while pregnant generally scored higher 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.

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.

Currently, U.S. health officials recommend that pregnant women eat no more than 12 ounces, or roughly two servings, of fish per week.

(Agencies via Xinhua News Agency April 8, 2008)