Diet recommendations under consideration by the U.S. government may lead pregnant mothers to eat unsafe levels of mercury, argues a new report from a consumer watchdog group.

At the same time, pregnant women may not be getting enough omega-3 fatty acids, part of the reason seafood is recommended during pregnancy.

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“The study shows that during pregnancy women should not only watch how much fish they eat, but what kind of fish,” according to the Environmental Working Group. “Pregnant women who follow the federal government’s draft dietary advice could eat too much fish high in toxic mercury, which is harmful to the developing brains of fetuses, babies and young children. There is strong evidence that mercury exposure during pregnancy and childhood causes lifelong deficits in learning, memory and reaction times."

Hair samples from 254 women from 40 states were tested. The women ate two or more seafood meals a week, as suggested by recommendations currently under consideration by the Food and Drug Administration and the Environmental Protection Agency, according to the study. Nearly a third of the women tested contained mercury levels the exceed EPA guidelines.

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Higher levels of mercury were found in women who consumed swordfish, marlin, shark and tuna steaks and tuna sushi. Lower-mercury species include catfish and tilapia, but the report notes they’re also lower in omega-3 fatty acids. Wild salmon, the study notes, is both high in omega-3 and low in mercury exposure. So are anchovies, herring, shad, sardines, Pacific oysters, trout and Atlantic and Pacific mackerel.

“Federal guidelines fall short on protecting women who are pregnant or planning to have children,” said Michael Bender, director of the Mercury Policy Project, in a statement released by the EWG. “Based on the evidence, it’s time for FDA and EPA to revise their advice, particularly when it comes to reducing tuna consumption, since it’s the largest mercury exposure in the American diet.”

The National Fisheries Institute disputed the results of the study, saying in a statement: "EWG recommends FDA bring its advice to pregnant women into alignment with the USDA Dietary Guidelines to, 'provide greater clarity.' What they do not mention is that the guidelines have historically said the 'benefits of consuming seafood far outweigh the risks, even for pregnant women.'"