MONDAY, Oct. 8, 2012 (HealthDay News) -- In another sign of the possible dangers lurking in an environmental hazard, new research links mercury exposure in expectant mothers to attention-deficit/hyperactivity disorder (ADHD) symptoms in their children at the age of 8.
The research doesn't prove that mercury is directly responsible for the behavioral problems. And it's not clear if the children in the study actually had ADHD, because the study only looked at symptoms, not diagnoses.
There's also a twist: Mercury is often found in fish, but those children whose mothers ate more fish during pregnancy appeared to have fewer behavioral problems, according to the report published online Oct. 8 in Archives of Pediatrics & Adolescent Medicine.
Overall, the study "adds to concerns about mercury consumption and to evidence about the benefits of fish consumption," said Dr. Susan Korrick, the study's co-author and an assistant professor of medicine at Harvard Medical School and Brigham and Women's Hospital in Boston.
Scientists consider mercury, which is used in many industries, to be a major environmental threat. It often works its way into the bodies of humans through fish. At the same time, fish can be healthy to eat.
In the new study, researchers sought to understand whether more mercury exposure before birth -- through mothers -- leads to more behavioral problems in kids.
The investigators looked at children at age 8 from the New Bedford, Mass., area who were born between 1993 and 1998. New Bedford is a fishing community, and fish consumption is popular.
The kids' mothers either had their mercury levels measured before birth (421 women in total) or answered questions about their fish consumption during pregnancy (515 women).
To determine whether the children showed signs of symptoms of ADHD, the researchers gave them tests and looked at reports from their teachers. The investigators found that some children -- of mothers who had the highest levels of mercury before birth -- were 40 percent to 70 percent more likely to have the behavioral problems. The researchers reached those numbers after adjusting their statistics so they wouldn't be thrown off by several factors, such as high or low numbers of kids of certain races and ages, and high or low numbers of parents of certain educational levels and habits (such as smoking).
The study doesn't prove that mercury levels in mothers and behavioral problems in children have anything to do with each other. There could be no connection at all, or an unknown third factor could explain both.
The design of the study didn't allow the researchers to identify the percentage of the children who actually have the behavioral problems. ADHD symptoms are seen in about 15 percent of the general population of kids, study co-author Korrick said.
Children of mothers who ate more than two servings of fish a week were actually more likely to not show signs of ADHD symptoms. But it's not clear what kinds of fish the mothers ate, nor whether those fish had high mercury content.
Pregnant women should avoid certain kinds of fish -- such as swordfish, fresh tuna and some kinds of tile fish -- because they have more mercury, Korrick said.
Dr. Bruce Lanphear, a health sciences professor who studies environmental risks at Simon Fraser University in Vancouver, British Columbia, said exposure to other substances -- such as lead, tobacco and pesticides -- appears to disrupt the prefrontal cortex, the most advanced part of the brain. When that happens, "you start changing the way people act and whether they can delay gratification and control their impulses. That should bother and worry us," he noted.
Lanphear also acknowledged that something other than mercury exposure could explain the higher risk of behavioral problems in the children in the study. But other research in animals suggests that the researchers are on the right track, he said.
For more about mercury, visit the U.S. National Library of Medicine.