A new study published this week in the journal Archives of Pediatrics & Adolescent Medicine shows that low-level prenatal mercury exposure might be associated with a greater risk of children having attention-deficit/hyperactivity disorder (ADHD). However, the study also showed that greater fish consumption might be associated with a lower risk of the child having ADHD.

The study was conducted by doctors at the Boston University School of Public Health and Brigham and Women’s Hospital. It looked at a group of infants born between 1993 and 1998 in the New Bedford, Massachusetts area. The cohort study compared maternal hair mercury levels in the last month of pregnancy and prenatal fish intake with the child’s ADHD-related behaviors at age 8.

The researchers found that inattention and impulsivity/hyperactivity appeared to be associated with prenatal mercury exposure. However, there also appeared to be what the researchers called a “protective” effect associated with fish consumption of greater than two servings per week.

As exposure to methylmercury comes mainly from fish consumption, this would seem to be a contradiction. The U.S. Environmental Protection Agency and U.S. Food and Drug Administration recommend that pregnant women eat no more than two six-ounce servings of fish per week. The study background indicates that nutrients found in fish, such as omega-3 fatty acids, have been shown to benefit brain development, “potentially confounding mucury-related risk estimates.”

“In summary, these results suggest that prenatal mercury exposure is associated with a higher risk of ADHD-related behaviors, and fish consumption during pregnancy is associated with a lower risk of these behaviors,” said the study’s authors. “Although a single estimate combining these beneficial vs. detrimental effects vis-à-vis fish intake is not possible with these data, these findings are consistent with a growing literature showing risk of mercury exposure and benefits of maternal consumption of fish on fetal brain development and are important for informing dietary recommendations for pregnant women.”