The study finds little evidence of harm in infants whose mothers had low fish consumption and low mercury exposure.
In fact, infants of mothers with higher mercury exposure during pregnancy and who consumed more fish had better attention and needed less special handling during a newborn exam. This likely was due to the beneficial nutritional effects of fish consumption, according to the researchers.
"The better neurobehavioural performance observed in infants with higher mercury biomarkers should not be interpreted as a beneficial effect of mercury exposure, which is clearly neurotoxic," says Kim Yolton, PhD, a researcher at Cincinnati Children's Hospital Medical Center and senior author of the study.
"It likely reflects the benefits of polyunsaturated fatty acid intake that also comes from fish and has been shown to benefit attention, memory, and other areas of development in children. In our study, mercury exposure was very low, primarily due to consumption of fish low in mercury, so the detrimental effects might have been outweighed by the beneficial effects of fish nutrition."
The study is published online in Neurotoxicology and Teratology.
The researchers assessed the neurobehaviour of 344 5-week-old infants using a standard neurobehavioural scale. Gestational mercury exposure was measured in maternal blood and infant umbilical cord blood. The researchers collected fish consumption information from the mothers and estimated polyunsaturated fatty acid intake based on the type and amount of fish consumed.
Eighty-four per cent of mothers reported eating fish during pregnancy but only about two ounces per week on average. Those infants with higher prenatal mercury exposure showed asymmetric, or unequal reflexes. But when fish consumption was taken into account, those whose mothers consumed more fish had better attention and needed less special handling.
In 2014, the FDA and EPA updated their advice to consumers to encourage women to eat more fish (eight to 12 ounces per week) than had previously been recommended and to select fish with the lowest mercury levels.
These include salmon, shrimp, pollock, light canned tuna, tilapia, catfish, and cod. They also suggested avoiding fish with the highest mercury levels, including tilefish, shark, swordfish, and mackerel.
"The important thing for women to remember is that fish offers excellent nutritional qualities that can benefit a developing baby or young child," said Dr Yolton. "Moms just need to be thoughtful about which fish they eat or provide to their child."
