Low-level mercury exposure in pregnant women connected to ADHD risk in children

Attention-deficit/hyperactivity disorder (ADHD) affects approximately ten percent of children worldwide, yet its causes are not well understood. Now, a study led by Susan Korrick, MD, MPH, of Brigham and Women's Hospital (BWH), and Sharon Sagiv, PhD, MPH, of Boston University School of Public Health, and published in the online version of the Archives of Pediatrics & Adolescent Medicine on October 8, 2012, links low-level prenatal mercury exposure with a greater risk of ADHD-related behaviors.

The study also finds that maternal fish consumption during pregnancy can help reduce the risk of ADHD-related behaviors in children. This duality is possible because many types of fish have low levels of mercury, so it is possible for a pregnant woman to eat nutritionally beneficial fish without being exposed to much mercury.

“These findings underscore the difficulties pregnant women face when trying to balance the nutritional benefits of fish intake with the potential detriments of low-level mercury exposure,” said Dr. Korrick.

Dr. Sagiv agrees this study provides an important public health message, saying, “Women need to know that nutrients in fish are good for the brain of a developing fetus, but women need to be aware that high mercury levels in some fish pose a risk.”
This analysis involved approximately 400 children born in New Bedford, Massachusetts between 1993 and 1998. Shortly after their mothers gave birth, researchers collected hair samples from the mothers and analyzed them for mercury. They also gave the mothers a questionnaire to determine their fish consumption during pregnancy. Eight years later, researchers followed up with the children and administered standardized tests to determine behaviors related to ADHD.

Researchers found an increased risk of childhood ADHD-related behaviors with increasing maternal hair mercury levels. These mercury levels were lower than levels shown to be potentially hazardous in most previous studies. Additionally, researchers found a reduced risk of ADHD-related behaviors in children whose mothers reported eating more than two servings of fish per week, which is a higher number of servings than is currently recommended by the United States Food and Drug Administration and Environmental Protection Agency.

The study did not examine what types of fish are best for a pregnant woman to eat, but previous studies have shown women should avoid fish that are high in mercury, such as shark, swordfish, king mackerel and fresh tuna. Fish that are low in mercury, such as flounder, haddock, and salmon, are safer to eat and good sources of nutrition.