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According to the U.S. Centers for Disease Control and Prevention, 1 in 11 school-aged children are diagnosed with Attention-Deficit/Hyperactivity Disorder (ADHD) and new research suggests that high levels of mercury during pregnancy may increase the risk of children with ADHD, while consumption of fish reduces chances of the disorder.

The new study has revealed that children born to women with highest levels of mercury were between 40 to 70 per cent more likely to show symptoms of ADHD by the time they are 8 years old.

The authors of the study, from Boston University, in America, said that although the primary source of mercury was from eating fish, consumption of more than two portions of fish per week by pregnant women decreased their chances of having children with attention and hyperactivity symptoms by 60 percent.

This implies the "difficulties of balancing the benefits of fish intake with the detriments of low-level mercury exposure in developing dietary recommendations in pregnancy", the researchers said according to Telegraph.

The possible explanation for the research findings could be that women with highest levels of mercury consumed fish high in the heavy metal, such as swordfish, shark and marlin, whereas most other women would have eaten fish low in Mercury such as sardines and mackerel.

However, according to some other experts, women with high levels of mercury in them could have been exposed to another source of mercury.

This is the first study to have linked eating fish in pregnancy to a lower risk of ADHD in children.

For the study, the researchers took hair samples of 421 mothers not long before they were about to deliver their child and asked them about their fish consumption frequency during pregnancy. Eight years later, the researchers followed up on their study participants and checked for the symptoms of ADHD in their children.

The researchers found that one microgram of mercury per gram of hair or more was associated with a rise in the chances of the child having ADHD symptoms. This level, say the researchers, was slightly lower than what the current recommended safe limit is.

 "This is an important paper that should be considered carefully. For me the most important take home message is that a mother eating more than 2 helpings of fish a week appears to reduce the risk of hyperactivity in her child by a considerable amount," Jean Golding, Professor of Paediatric and Perinatal Epidemiology at University of Bristol was quoted as saying by The Telegraph.

"The other message is that high levels of mercury in the mother have some adverse consequences for the child's behavior - but this must imply that it is not the mercury from seafood that is the culprit. This is a complex field, but important to untangle. Meanwhile the take home message for pregnant mothers is that eating fish is good for the future behavior of her child. "

"We're aware of the paper which needs to be considered alongside the existing literature on this issue. The European Food Safety Authority is currently reviewing the risks to health from mercury and methylmercury and is expected to publish its preliminary opinion later this year. The FSA's advice remains that risks from mercury in fish are mainly an issue for pregnant women and women who intend to become pregnant. This is because of the possible risks to the developing nervous system of the unborn child. Our advice is that pregnant women and women intending to become pregnant shouldn't eat more than four medium-sized cans or two fresh tuna steaks per week. They should also avoid shark, marlin and swordfish," a spokesman for the Food Standards Agency said according to the report.

As many as 40 percent of children have significant problems with attention by age four, and ADHD is now the most common mental health disorder diagnosed in the preschool years.

The research was published in the journal Archives of Pediatrics and Adolescent Medicine.