MONDAY, Sept. 29 (HealthDay News) -- Greater maternal consumption of fish and longer periods of breast-feeding are tied to better physical and cognitive development in infants, according to a new study.

The report, which looked at mothers and infants from Denmark, provides further evidence that the omega-3 fatty acids found in fish and compounds in breast milk aid infant development.

"These results, together with findings from other studies of women in the U.S. and the United Kingdom, provide additional evidence that moderate maternal fish intake during pregnancy does not harm child development and may on balance be beneficial," study lead author Emily Oken, an assistant professor at Harvard University, said in a university news release.

Researchers from the Maternal Nutrition Group at the Department of Epidemiology at Statens Serum Institut in Copenhagen, Denmark, also collaborated on the study, which was published in the September issue of the American Journal of Clinical Nutrition.

The research, which looked at 25,446 children born to mothers participating in a Danish study between 1997 and 2002, found that children whose mothers ate the most fish during pregnancy (about 2 ounces a day on average) were more likely to have better motor and cognitive skills. Meanwhile, those whose mothers ate the least fish had the lowest developmental scores at 18 months of age.

Children who were breast-fed for longer periods of time also scored better, especially at 18 months. Breast milk also contains omega-3 fatty acids. The benefit of fish consumption was similar among infants breast-fed for shorter or longer durations.

U.S. women are advised to limit their fish intake to two servings a week because some fish contain high traces of mercury, a known toxin. Most women in the study, however, consumed cod, plaice, salmon, herring and mackerel -- fish that tend to have low-mercury levels.

"In previous work in a population of U.S. women, we similarly found that higher prenatal fish consumption was associated with an overall benefit for child cognitive development, but that higher mercury levels attenuated this benefit," Oken said. "Therefore, women should continue to eat fish -- especially during pregnancy -- but should choose fish types likely to be lower in mercury."