A new study has found that woman who eat mercury-containing fish during pregnancy have a higher risk of having a child with attention deficit hyperactivity disorder. Researchers say that mothers-to-be should eat fish, but must avoid fish that contain high levels of mercury.

Attention deficit hyperactivity disorder (ADHD) is a common childhood disorder. Children who have ADHD have difficulty controlling their actions and paying attention.

Methyl mercury can be found in contaminated fish and cooking does not reduce the risk of mercury exposure. Recently, a health group recommended that tuna fish should be removed from school lunchrooms after high levels of mercury had been detected in these fish.

Fish consumption has been promoted for a variety of health and cognitive benefits. Some studies have shown that children living in regions that have a higher fish consumption do better at cognitive tests. Experts say that certain fatty acids help protect against the harmful effects of mercury.

"Women need to know that nutrients in fish are really important for brain development, but they also need to be aware that high mercury levels in some fish pose a risk," said Sharon Sagiv, PhD, one of the study authors.

The U.S. Environmental Protection Agency has recommended that pregnant women must limit their total fish intake to no more than two 6-ounce servings per week. The new study says that a mother can increase the levels of fish consumption provided she is eating the fish that have high mercury levels.

"The message is, 'Eat fish -- but don't eat fish high in mercury,' " such as swordfish, shark, fresh tuna or king mackerel. Fish that are low in mercury include flounder, haddock and salmon," Sagiv said, according to a press release.

The present study included more than 400 children born between 1993 and 1998. Immediately after the children's birth, the researchers measured the levels of mercury in the body. Mothers were also asked to complete a questionnaire about their fish intake during pregnancy.

After eight years, the researchers analyzed the children's behavior for symptoms of ADHD.

The researchers found that the ADHD symptoms in the children were related to the mercury levels found in the mother's body. However, the levels of mercury that were detected in the women were less than what is considered to be dangerous. This means that even low level of mercury exposure was linked to high risk of developing ADHD.