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Mercury exposure while pregnant may increase baby’s risk of ADHD

Being exposed to mercury while pregnant may increase the risk of attention deficit hyperactivity disorder (ADHD) in your child, a new study has found. But eating fish while pregnant may lower your child’s risk of this condition.
By David McNamee

BMJ Group News

What do we know already?


Mercury is a neurotoxin, a substance which damages the nerve cells in the brain. Some studies have suggested that there may be a link between mercury exposure in pregnant women and ADHD. But there haven’t been many reliable studies done.

ADHD is a common developmental disorder in children. A child with ADHD finds it very difficult to sit still or concentrate, or to think before acting. All children find this difficult to an extent, but in children with ADHD their symptoms cause severe problems at home and at school.

The most common way of being exposed to mercury comes from eating fish. In some countries, such as the UK and the United States, the government recommends pregnant women limit their fish intake to no more than two servings a week. However, we also know that nutrients found in fish, such as omega-3 fatty acids, boost brain development in babies in the womb.

The researchers behind this new study wanted to see whether the benefits of eating fish offset the risk of ADHD in children whose mothers were exposed to mercury while pregnant. The study followed 788 babies born between 1993 and 1998 in the US. Researchers examined hair samples from mothers in the last month of pregnancy and tested them for mercury, and asked the women how much fish they ate. The researchers then examined the children born during the study at the age of eight years old for symptoms of ADHD such as having difficulty paying attention, acting impulsively, and being hyperactive.

What does the new study say?

At lower levels of mercury exposure, the benefits of the nutrients in the fish eaten by the pregnant women outweighed the potential harm caused by the mercury in the fish. However, at higher levels of exposure, the benefits of eating the fish may be outweighed by the potential harm caused by the mercury.

ADHD symptoms were more common in the children of mothers who were exposed to higher levels of mercury than the US Environmental Protection Agency’s recommended cut-off point of 1.2 milligrams of mercury per gram of fish. This is roughly the same as the amount of mercury you would be exposed to if you ate two 6-ounce servings of fish every week.

The children of women who ate more than two servings of fish a week were less likely to have symptoms of ADHD. But the study also suggests that the more mercury you are exposed to the less benefit you may get from eating fish. So the link between mercury exposure, eating fish and behavioural problems is not a simple one.

How reliable is the research?

This was a well-conducted study, although the results contradict findings in studies from other parts of the world. This may have been because the levels of nutrients varied in different types of fish, so the benefits of eating fish were not always the same.

We can’t be sure that people gave accurate details about how much fish they ate, but testing their hair for mercury makes this measure more reliable. This type of study can’t prove that there’s a link between being exposed to mercury while pregnant and having a child with ADHD.

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