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03 October 2005 - Harvard Medical School - Fish Consumption by Pregnant Women May Increase Cognitive Ability in Infants

Report in Environmental Health Perspectives finds health benefits, but concerns remain about mercury levels

Recent recommendations by the FDA advising pregnant women to limit mercury-containing fish in their diets may have the unintended consequence of depriving fetuses of essential nutrients, according to a study published today in the October issue of the peer-reviewed journal Environmental Health Perspectives (EHP). Although excessive mercury intake during pregnancy can harm the neurological development of fetuses, today's study found that nutrients in fish, such as n-3 polyunsaturated fatty acids, may play a critical role in an infant's neurocognitive development.

Researchers tested six-month-old infants' cognitive ability and compared it to both the amount of fish consumed by the mother during pregnancy and the amount of mercury found in the mother's hair. As had been found in previous studies, elevated maternal mercury levels were associated with a deficit in infant cognition. However, higher fish intake was associated with higher infant cognition, especially after adjusting for mercury levels.

While these results may seem contradictory, researchers found that the infants who scored highest on cognitive tests were those whose mothers ate more fish and had lower levels of hair mercury.

"The most likely explanation is that the benefit is conferred by consuming fish types with the combination of relatively little mercury and high amounts of beneficial nutrients," wrote the authors of the study. Fish that tend to be higher in n-3 fatty acids but lower in mercury include salmon, canned light tuna, and sardines.

The study, conducted by researchers at Harvard Medical School, examined data from 135 woman-infant pairs who participated in Project Viva, a prospective pregnancy and child health cohort study.

The women completed a food frequency questionnaire that recorded how often during the second trimester of pregnancy they ate four different fish types (canned tuna, shellfish, dark meat fish such as salmon, and all other fish). Maternal hair samples collected at delivery provided a separate measure of mercury intake during the second trimester.

The final stage of research called for infant cognitive testing conducted at approximately 6 months of age. Infants took the visual recognition memory test, which analyzes the child's ability to recognize an initial stimulus and record into memory a novel stimulus. This test has been shown to correlate to IQ later in life.

The researchers sought to better understand the sometimes opposing opinions regarding the consumption of fish by pregnant women. "The net effect of the beneficial nutrients and harmful contaminants contained within fish has not been well studied and remains unclear," they wrote. The study authors suggest that future research include more detailed dietary information to help pregnant women make informed decisions about which fish species may be better or worse for their child's cognition.

The lead author of the study was Emily Oken of the Department of Ambulatory Care and Prevention at Harvard Medical School. Other authors included Robert O. Wright, Ken P. Kleinman, David Bellinger, Chitra J. Amarasiriwardena, Howard Hu, Janet W. Rich-Edwards, and Matthew W. Gillman. Funding for the research was provided by the National Institutes of Health, Harvard Medical School, and the Harvard Pilgrim Health Care Foundation. The article is available free of charge at