Pregnant women who want to eat the best diet for their babies worry about fish.

Fish are rich in DHA, an omega-3 fatty acid essential to brain growth, but many of the most commonly eaten fish are contaminated by mercury, a toxin that can damage the developing nervous system of a fetus and young child. As a result, the federal government advises pregnant women and nursing mothers to avoid fish with high mercury levels and eat no more than two servings a week of other fish.

Results from a Harvard-led study conducted in Denmark tip the scales in favor of eating more fish. The benefits of eating fish low on the mercury list outweigh the risks, they said in a paper published in the American Journal of Clinical Nutrition.

Working with Danish researchers, the Harvard team tracked more than 25,000 women, asking them what they ate and how long they breastfed their babies. (Breast milk is also high in DHA, so its influence had to be measured and taken into account.) When the babies were 6 months old and again at 18 months old, the mothers answered questions about developmental milestones, from sitting up at 6 months to putting words together at 18 months.

 

Mothers who ate more fish while they were pregnant and breastfed their babies longer than other mothers had children who showed better physical and cognitive development when they were 6 months old and again at 18 months old, the study shows. Each practice alone was helpful.

Women who ate at least three servings of fish per week had children who were 25 percent more likely to score high on development at 6 months and 30 percent more likely to score high at 18 months than children of mothers who ate less than one serving of fish per week.

The authors note that the kind of fish most of the Danish women ate -- cod, plaice, salmon, herring, and mackerel -- are likely to have low mercury content. Shark and king mackerel, which have high mercury levels, are not commonly eaten in Denmark. The US Food and Drug Administration has information on mercury levels.

"These results, together with findings from other studies of women in the United States 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," lead author Dr. Emily Oken said in a statement. "Women should continue to eat fish -- especially during pregnancy -- but should choose fish types likely to be lower in mercury."