Advice given to women on the amount of fish they should consume while pregnant may be flawed, researchers contend.

Experts agree that fish is a good source of protein and healthy fats, but these health benefits must be balanced with significant downsides. Fish species like tuna and carp, can contain high levels of mercury, which are absorbed from polluted waters.

Previous studies have shown that mercury, a heavy metal, can be toxic to neurons in the brain, and in 2014 U.S. federal agencies issued draft guidelines advising pregnant women or women trying to get pregnant, to avoid high-mercury fish altogether.

However, scientists at the Environmental Working Group (EWG) argue that even if women follow the guidelines on which types of fish to eat, they may be exposing themselves and unborn babies to unsafe levels of mercury and not gaining the necessary benefits from the healthy omega-3 fats in the fish.

The EWG claims the list of high-mercury fish is incomplete or inaccurate. Specifically, canned light tuna is listed as a lower-mercury fish even though some previous studies have found it high in mercury.

"It’s misleading to name canned light tuna as one of the low-mercury species that women are encouraged to eat,” says Sonya Lunder, senior analyst at EWG.

EWG asked 254 women of child-bearing age who ate more than the U.S. government’s recommended amount of fish to record their seafood consumption and submit hair samples for mercury testing.

Among women following this preliminary advice of two to three servings of different types of fish a week, 30 per cent of the women were exposed to levels of mercury deemed unhealthy by the EPA.

Much of their exposure was tied to fish species like tuna steaks and sushi that are not included in the government’s warning.

The FDA guidelines, which aren’t final and still in draft form, only mention swordfish, king mackerel, tilefish and shark as species for pregnant women to avoid, and suggest limiting albacore tuna to six ounces per week.

To avoid mercury exposure, the guidelines provide a list of low mercury seafood, which include salmon, anchovies, herring, shad, sardines, Pacific oysters, and trout.

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