Learning and Behavior in Adolescents Whose Mothers Ate Fish During Pregnancy

Persistent worries about the potential dangers from consuming fish and the methylmercury they contain continue to frighten women away from eating fish during pregnancy, especially in the U.S. Mercury is present in nearly all seafood in the form of methylmercury. Seafood consumption is already very low in the U.S. and Canada, as shown in government surveys and research studies. Some estimates suggest that as many as 20% of women of child-bearing age eat no fish or shellfish. This poses a problem for women during pregnancy and lactation. Women who eat no fish or very little seafood and do not take fish oil supplements have almost no dietary source of the long-chain omega-3 fatty acids found in seafood. These fatty acids are critical for the healthy development of the brain and retina of the fetus. It also means these mothers have smaller body stores of these omega-3s for the fetus to draw upon and to provide sufficient amounts in their breast milk. With very few exceptions—shark, king mackerel, tilefish and swordfish—most fish in the seafood marketplace has very low levels of contaminants, including methylmercury. Many studies have reported that women who eat plenty of commercially available fish during pregnancy have offspring with significantly lower chances of having suboptimal developmental scores on a variety of test measures. In other words, their children perform better and do not have any signs of harm from low-level exposure to mercury. One of the strongest reasons for asserting that eating fish during pregnancy benefits, not harms, the offspring comes from the long-term research study conducted in the Seychelles Islands. People there consume 10 times as much fish and shellfish as eaten in the U.S. and their children show no clinically meaningful detriments from such a diet at all stages of development. Yet mothers in the Seychelles have higher hair mercury levels and dietary intakes than women in the U.S. The most recent study from the Seychelle Islands reports the cognitive and behavioral test results in the adolescent offspring of mothers who have been monitored since pregnancy. These offspring were 17 years of age, on average. They underwent a series of learning tests and behavioral assessments to evaluate their learning abilities and social behaviors. These included such aspects as mental health, antisocial behavior, substance abuse and referrals to a school counselor. The test findings were then analyzed for their relationship to prenatal methylmercury exposure and adjusted for confounding (interfering) factors such as maternal intelligence and sex. The investigators found that higher exposure to prenatal methylmercury was associated with better performance on the achievement scores, suggesting that the students had enhanced problem-solving ability. Similarly, the adolescents with the highest methylmercury exposure had improved scores on several behavioral assessments. For example, there were fewer substance abuse reports, especially for females with the highest number of reports. Overall, learning and behavior scores were either higher in the students exposed to greater methylmercury levels, or there was no link between scores and prenatal mercury exposure. Of the 27 evaluations given, only one adverse association was observed and that was for the lowest category of referrals to a school counselor. There was no association between methylmercury exposure and the more troublesome category of frequent referrals. There is no reason to think that methylmercury itself was contributing to the improved performance of these students. In all likelihood, methyl mercury exposure reflects greater fish consumption and higher nutrient intakes, especially of seafood omega-3s, selenium, iron, iodine and others especially advantageous in brain development. This long-running study on the outcomes in mothers and their offspring according to their environmental exposures to fish and contaminants has reported over 70 different endpoints for learning, achievement and problem behaviors. To date there has been no consistent pattern of adverse or undesirable associations between prenatal methylmercury exposure and developmental outcomes. These findings provide substantial reassurance that eating plenty of fish during pregnancy and the exposure to methyl mercury that goes along with it is not linked to harmful effects in the offspring. Eating fish and shellfish during pregnancy is one of the easiest ways to obtain critical nutrients that promote healthy fetal and infant development.