Building Babies' Brains - Yes To Fish, No To Mercury

by Karen Switkowski, MPH, MS

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 “I really want to get the salmon, but I can’t remember how many times I already had fish this week. Did we have that shrimp pasta on Sunday, or was that Saturday? I guess I’ll be OK if I have this now and we don’t make the tuna until next weekend…” Throughout my pregnancy, my husband had to listen to some version of this monologue every time we went out to dinner. Like many women, I had internalized the warning that I risked poisoning my baby with mercury if I ate seafood too often. Although I was also well aware of the importance of the nutrients found in fish and other seafood for fetal development, I was more concerned about consuming too much mercury that I was about not getting enough beneficial fatty acids in my diet.

Omega-3 long chain polyunsaturated fatty acids (n-3 PUFA), including those commonly referred to as ALA, EPA, and DHA, are crucial for development of the placenta and fetal brain. While fish and seafood are the major dietary source of n-3 PUFA, these foods may also be contaminated with methyl mercury, which is toxic to the fetal brain.

While women may be protecting their developing babies from mercury exposure by avoiding intake of fish and seafood, they may also be consuming too few n-3 PUFAs for optimal fetal brain development.  A study conducted by OPP’s Dr. Emily Oken in our Project Viva cohort demonstrated that second-trimester fish consumption was positively associated with measures of infant and child cognition, after adjustment for maternal mercury levels. Similar observational studies conducted in the UK, Spain and Denmark have indicated that higher intake of dietary sources of n-3 PUFA during pregnancy is associated with improved cognitive and behavioral development in the offspring.

The 2015 Dietary Guidelines Advisory Committee conducted a risk/benefit analysis of prenatal exposure to the beneficial and harmful components of seafood and concluded that “the neurodevelopmental risks of not eating fish exceeded the risks of eating fish.” However, some fish accumulate higher levels of mercury and are best avoided during pregnancy. The new 2015 Dietary Guidelines confirm earlier recommendations by the FDA and EPA that pregnant and breastfeeding women avoid consuming tilefish, shark, swordfish, and king mackerel due to high methyl mercury content. Most of the recommendations that I came across in books and handouts provided by my OB clinicians suggested limiting consumption of low-mercury fish and seafood to an average of 2 servings per week during pregnancy.

Although dietary guidelines established by experts in the US, Canada, Australia and Europe all recommend that pregnant women regularly consume fish during pregnancy, studies of women in all of these areas indicate that intake is well below that required to obtain adequate levels of n-3 PUFA. Are the warnings about mercury exposure discouraging women from eating enough fish? Dr. Oken’s work suggests that this is in fact the case. A 2003 study of Project Viva participants indicated that pregnant women reduced their fish consumption following a 2001 federal advisory recommending limited fish intake due to concerns about mercury contamination.

Personally, I found that warnings about the dangers of mercury during pregnancy were much more effectively disseminated than information about the importance of n-3 PUFA. Given the recent evidence indicating that a diet deficient in n-3 PUFA may be more of a threat to fetal neurodevelopment than exposure to small amounts of mercury,it may be time to rethink the public health and clinical approaches to educating pregnant women about fish consumption.