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A new study suggests that prenatal exposure to mercury is associated with symptoms of attention deficit hyperactivity disorder, but the greater a mother’s consumption of fish — a source of mercury — the less likely her child is to suffer these symptoms.

The apparently paradoxical findings, published online last week in Archives of Pediatrics & Adolescent Medicine, come from an analysis of 607 children born between 1993 and 1998. The researchers reviewed data on the amounts of mercury in the mothers’ hair, comparing them against dietary records. At ages 7 to 10, the children underwent neuropsychological examinations.

After controlling for fish consumption and many other factors, the scientists found an association between several A.D.H.D.-related behaviors and levels of mercury above one microgram per gram in the maternal hair samples.

At the same time, they found that after adjusting for mercury levels, mothers who ate more than two servings of fish per week — more than the 12 ounces that government guidelines suggest — were less likely to have children with A.D.H.D.-related behaviors.

“All fish has some mercury in it, but there are very different levels,” said the lead author, Sharon K. Sagiv, an assistant professor at Boston University.

The findings may seem contradictory, she added, but “they highlight an important public health issue: Eating fish is good for you, but eating fish that is high in mercury is not.”