Emily Oken of the Harvard Medical School and Harvard Pilgrim Health Care and colleagues looked at 25,446 children born to mothers participating in the Danish Birth Cohort, a study that includes pregnant women enrolled from 1997 to 2002.

Prenatal diet, including amounts and types of fish consumed weekly, was assessed by a detailed food frequency questionnaire.

The study, published in the the American Journal of Clinical Nutrition, found children whose mothers ate the most fish during pregnancy were more likely to have better motor and cognitive skills. Compared with women who ate the least fish, women with the highest fish intake -- about 2 ounces per day on average -- had children 25 percent more likely to have higher developmental scores at six months and almost 30 percent more likely to have higher scores at 18 months.

Consumption of three or more weekly servings of fish was associated with higher development scores, so the nutrient benefits of prenatal fish appeared to outweigh toxicant harm, the researchers said.

However, pregnant women in the United States have been advised by the federal government to be careful about the type of fish they eat because of concerns about mercury levels in some species.