Maternal Fish Intake During Pregnancy, Blood Mercury Levels, and Child Cognition at Age 3 Years in a US Cohort

While fish (and other seafood) may contain beneficial nutrients, such as n-3 fatty acids, they may also contain potentially harmful substances such as mercury. The fetal brain is especially sensitive to the neurotoxicant methylmercury. In this prospective cohort study, which enrolled 341 mother-child pairs in the years 1999–2002, associations were sought between estimated second-trimester fish intake, red blood cell mercury concentrations, and children's scores at age 3 years on 2 measures of cognitive function: the Peabody Picture Vocabulary Test (PPVT), and the Wide Range Assessment of Visual Motor Abilities (WRAVMA). Participants completed a pregnancy-modified semiquantitative food frequency questionnaire when seen in the second trimester. Maternal fish intake averaged 1.5 servings per week. While 12% of mothers had more than 2 weekly servings of fish, 14% ate no fish at all. Fish intake correlated directly with total erythrocyte mercury levels. On multivariable linear regression analysis, higher levels of fish intake were associated with better performance on cognitive testing, while higher mercury levels correlated with poorer test scores. Effect estimates for high fish intake, compared to the absence of fish from the diet, were 2.2 (95% confidence interval [CI], −2.6, 7.0) for the PPVT and 6.4 (95% CI, 2.0, 10.8) for the WRAVMA. For mercury levels in the highest decile, effect estimates were −4.5 (95% CI, −8.5, −0.4) for the PPVT and −4.6 (95% CI, −8.3, −0.9) for the WRAVMA. Eating fewer than 2 weekly servings of fish was not associated with cognitive benefit. The associations found in this study were stronger when both fish intake and mercury levels were taken into account. When formulating recommendations for fish consumption by pregnant women, not only the potential harm from mercury exposure but, in addition, the possibility of nutritional benefits such as improved cognitive test scores in early childhood, as documented in this study, should be considered.