Despite concerns that mercury in fish might endanger a developing fetus, the amount of fish a mother eats appears to pose no harm to the future mental health of her child, according to a new study. Mercury occurs naturally in the environment and as a by-product of industrial pollution. It can accumulate in certain long-lived fish that consume other fish. The Food and Drug Administration advises pregnant women to avoid eating shark, swordfish, mackerel etc. because they may contain high levels of mercury, which can harm the developing foetal nervous system. Women in the Seychelles ate a diet with a rich variety of fish, some of which contained high levels of mercury. Researchers at the University of Rochester in New York studied a sample of almost 800 mothers and their children living in the Seychelles, where people eat fish an average of 12 times each week. They measured mercury levels in hair samples taken from these women who had just given birth. Nine years later, the researchers performed tests of mental functioning in their children, and compared the results. Overall, how well children performed on the tests showed no relationship to the amount of mercury present in their mothers' bodies during pregnancy, with few exceptions, the report indicates. Children whose mothers had more mercury tended to perform more poorly on a particular motor skills test. It is unclear how much mercury one would need to be exposed to affect the fetus at this point, but it appears it would have to be above what these women were eating. Although previous results have indeed shown that high levels of mercury can harm the brain, tests involved much larger doses of mercury. And at high enough doses, even table salt is toxic and most substances are poisonous to some degree. The amount of fish women eat during pregnancy likely poses no overall harm to the future mental health of their children. There is no reason for pregnant women to reduce fish consumption below current levels, which are probably safe.

The Lancet , May 2003; Vol. 361