The researchers concluded that pregnant women should eat fish because their babies are likely to score higher on intelligence tests. But they also reported that the benefits of the nutrients disappear and the babies' intelligence scores drop substantially if the fish contains high levels of mercury.

For each additional weekly serving of fish, the babies' intelligence scores increased by 4 points, or an average of almost 7%. But for every increase of 1 part per million of mercury, the babies' intelligence scores dropped by 7.5 points, or 12.5%. A woman could raise her mercury level by 1 ppm if she ate an average-sized serving of swordfish once a week, said Dr. Emily Oken of Harvard Medical School, the study's lead researcher.

The FDA entirely rules out swordfish and shark as well as king mackerel and tilefish, found on the Atlantic Coast and Gulf of Mexico, for pregnant and nursing women and young children. Some white and albacore tuna, canned and fresh, also have high mercury levels. Generally, the darker the fish meat, the higher the mercury content.