Fish accounted for just 7 percent of the mercury measured in a group of pregnant women’s blood, found a new British study, whose authors argue that the benefits of eating fish during pregnancy appear to outweigh the costs.

And while those conclusions have received a fair dose of media attention with pro-fish headlines, other researchers continue to urge caution.

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Among the study’s shortcomings, said environmental epidemiologist Jaymie Meliker, the women included in the sample had relatively low levels of mercury overall. The study also failed to separate out the contribution of different species of fish, even though some varieties are known to contain more mercury than others.

“In essence, these researchers are saying that because they saw low levels of blood mercury in their population, mercury in fish is not a large problem,” said Meliker, of Stony Brook University in New York. But, he added, “there is strong evidence to the contrary from many studies in other populations.”

“It’s pretty clear that people who eat more fish, especially more high-mercury fish, have higher levels of mercury in them,” he said. “That’s been well established for at least 20 years.”

Mercury gets into the environment from coal-burning power plants, cement plants, steel production and other industrial sources. Once the environmental contaminant gets into oceans and rivers, according to the National Resources Defense Council, it accumulates in fish, including many that we eat.

In our bodies, mercury targets the nervous system and possibly the cardiovascular system, and developing fetuses are particularly vulnerable. Case studies have linked high levels of mercury exposure to infertility and miscarriage in pregnant women. Babies exposed in the womb can develop serious disabilities, including trouble with memory, attention and other thinking tasks.

Based on what we know so far, the United States Environmental Protection Agency has set safe mercury blood levels at 5.8 micrograms per liter. Still, there are many unknowns, including questions about the source of mercury in our bodies and the effects of long-term, low-dose exposures.


To test the long-held assumption that seafood accounts for the majority of mercury in human blood, researchers turned to data collected as part of an ongoing, long-term study called Children of the 90s. With an overall goal of understanding how the environment influences health and development, the study includes nearly 14,000 British pregnant women who delivered their babies in 1991 and 1992.

Previous analyses found that women who eat fish during pregnancy have children who are more intelligent and have better eyesight, said Jean Golding, a pediatric and perinatal epidemiologist at the University of Bristol in the United Kingdom.

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For the new study, Golding and colleagues compared blood mercury levels with self-reports of what women were eating during pregnancy for nearly 4,500 of the mothers.

Out of more than 100 different types of food and drink, statistical analyses showed that seafood turned out to contribute just 7 or 8 percent of the mercury in those mothers, the researchers reported in the journal Environmental Health Perspectives. Wine and especially herbal teas were far bigger dietary sources of mercury.

“If people are worried about mercury -- and certainly, high levels of mercury in the blood are not good for the fetus -- stopping eating fish won’t make that much of a difference to total blood mercury levels of the mother,” Golding said. “And we’ve shown that eating fish is actually so beneficial that people should not be put off by mercury.”

There are definitely health benefits to eating fish, Meliker agreed. But it may not be safe to generalize the new results to all women. Most of the women in the new study had levels that fell below the safe dose, and the researchers separated fish only into “white” and “oily” varieties.

If they had looked specifically at the contributions of high-mercury fish like swordfish or ahi tuna and if they had looked at populations with larger doses of mercury exposure, they may have seen a different picture.

“I think it’s very good to eat fish,” Meliker said. “To protect pregnant women, and people in general, the best recommendation is to eat lots of low-mercury fish.”

For specific fish advice, see these recommendations for low-mercury fish and other resources.