Canned tuna is known to contain low levels of mercury, but a new Consumer Reports investigation has found spikes of the neurotoxin in some cans.
The organization tested five popular tuna brands, CBS News reported.
While the mercury levels were all within U.S. Food and Drug Administration standards, Consumer Reports urged pregnant women to "avoid canned tuna altogether."
FDA guidelines say pregnant women can eat canned tuna in limited quantities.
"While canned tuna, especially light varieties, has relatively low average levels of mercury, individual cans can sometimes have much higher levels," Consumer Reports said.
"From can to can, mercury levels can spike in unpredictable ways that might jeopardize the health of a fetus," said James Rogers, director of food safety research and testing at the independent nonprofit.
Mercury can affect neurodevelopment, said CBS News medical contributor Dr. David Agus.
The effects may include impaired brain function and developmental delays in children. If a fetus is exposed to high levels of mercury, it may lead to thinking and memory issues later on, he said.
"Young children and pregnant women especially need to keep mercury away from those neurons that are developing," Agus told CBS News.
Consumer Reports tested 10 tuna products from five brands: Bumble Bee, Chicken of the Sea, Safe Catch, StarKist and Wild Planet. The tests included about 30 samples of both albacore and light tuna from each brand, all packed in water, CBS News reported.
Light tuna tended to have less mercury than albacore, the tests showed.
"But you can't tell by just looking how much mercury a specific can has," said Consumer Reports senior scientist Michael Hansen.
Consumer Reports found six individual spikes in mercury content among the 30 samples "that would change the FDA's recommendation about how often someone should eat that particular tuna," CBS News reported.
Consumer Reports said kids should eat only light and skipjack tuna in limited quantities and adults who aren't pregnant should aim for 8 to 12 ounces per week of fish that is lower in mercury.
"That could include up to three servings of light or skipjack tuna. ... You can eat albacore, but only one 4-ounce serving per week," Consumer Reports said.
The tests provide information on what consumers "may experience at a moment in time when eating these brands of tuna, and underscore the importance of making safer choices in their daily routines," Consumer Reports said.
"One big takeaway is that albacore has much more mercury than light or skipjack tuna, regardless of the brand," the organization said. "That's not surprising, since albacore is larger and lives longer than the tunas that make up the light tuna or skipjack tunas. But the disparity was quite wide: The albacore products had three times more mercury, on average, than the others."
Mercury is an impurity in coal. Elemental mercury goes to the clouds when coal is burned and then comes down when it rains, Agus told CBS News. Marine animals consume it when it ends up in the ocean as methylmercury.
It can't be removed through cooking.
While higher levels of mercury appear in shark and swordfish, trout and salmon contain little.
Four of the five tuna companies told CBS News their products are safe and within FDA standards. Bumble Bee did not immediately respond, but told Consumer Reports that "health benefits of consuming seafood far outweigh any potential risk, including concerns about mercury," CBS News reported.