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 per cent of the study scored more poorly than those whose mothers had lower mercury levels.

Only 2 per cent 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.