Children exposed to low levels of mercury in the womb are more likely to exhibit behaviors consistent with attention deficit/hyperactivity disorder, a new study finds.

The finding supports concerns about mercury that have led to recommendations from the U.S. Food and Drug Administration and other agencies that pregnant women limit their intake of fish to no more than two six-ounce servings each week. But the same study also found that kids whose mothers ate more fish during pregnancy than recommended had with a lower risk for those same behaviors.

While fish is the primary source of dietary methylmercury, it is also packed with lots of brain-benefiting nutrients like omega-3 fatty acids.

A team of researchers led by Boston University School of Public Health researcher Sharon Sagiv examined data from the New Bedford birth cohort, a group of infants who were born between 1993 and 1998 to mothers that lived near a contaminated harbor in Massachusetts. They published their results on Tuesday in the journal Archives of Pediatrics & Adolescent Medicine.

Hair samples taken from 421 mothers during the last month of pregnancy or shortly after birth were analyzed for mercury levels, and 515 mothers reported how much fish they ate during gestation.

Mercury exposure during pregnancy appeared to be associated with inattention, impulsivity and hyperactivity in children. But eating more than the two recommended fish servings per week was actually negatively correlated with those same ADHD-related behaviors, particularly impulsivity and hyperactivity, according to the paper.

To sum up, “low-level prenatal mercury exposure is associated with a greater risk of ADHD-related behaviors, and fish consumption during pregnancy is protective of these behaviors,” the authors wrote. “These findings underscore the difficulties of balancing the benefits of fish intake with the detriments of low-level mercury exposure in developing dietary recommendations in pregnancy.”

In an accompanying editorial, Simon Fraser University researcher Bruce P. Lanphear said the study from Sagiv and her colleagues has many implications.

“First, we can take some comfort in recent legislation to reduce mercury contamination, at least from domestic sources. Second, these studies should spur our efforts to enhance the collection of data needed to calculate national estimates and trends in ADHD," Lanphear wrote.

Lanphear also said the study underscores the need for a national scientific panel to examine the environmental factors that influence the development of ADHD.

“This study and a flurry of new evidence linking environmental contaminants with ADHD reinforce the urgency of revising the regulatory framework for environmental contaminants and toxicants," Lanphear said.

SOURCE: Sagiv et al. “Prenatal Exposure to Mercury and Fish Consumption During Pregnancy and Attention-Deficit/Hyperactivity Disorder-Related Behavior in Children.” Arch Pediatr Adolesc Med. Published online 9 October 2012.