They are questions every pregnant woman must resolve: how much and which types of fish to consume?

Eat too much of the wrong kind, and you may end up dosing your unborn baby with excess mercury, a toxin known to impair fetal brain development.

Eat too little fish, though, and you'll deprive him or her of precious omega-3 fatty acids, which are known to enhance not only brain development but also fetal growth in general.

Last week, the Food and Drug Administration and the Environmental Protection Agency issued finalized guidelines aimed at resolving this issue. 

The new recommendations—which apply to pregnant and breastfeeding women as well as all women of childbearing age (16 to 49 years old) and young children—come after years of hand-wringing debate.

Consumer Reports’ food safety experts say that overall, these new guidelines are a vast improvement over both previous guidelines and recently proposed ones. “The new advice on fish is much more protective of women and their babies and children,” says Jean Halloran, director of food policy initiatives for Consumers Union, the policy and mobilization arm of Consumer Reports.

But in reviewing the recommendations, Halloran says there are some details the CR food safety team disagrees with. “We don’t think that all of the fish on the agencies’ ‘best’ or ‘good’ choices lists belong there—especially canned light and albacore tuna," she says.

Here's a breakdown of what these latest guidelines advise and what Consumer Reports thinks you should do:

Many Improvements

The new recommendations reflect several positive changes from the guidelines the agencies originally proposed in 2014.

Those original guidelines set minimum and maximum weekly quotas for fish consumption—8 to 12 ounces. But aside from telling the at-risk groups to avoid certain very high-mercury fish, the agencies adopted what is called the “net benefits” rationale. “Net benefits" simply means that any fish should be deemed safe to eat if the benefits of omega-3s outweigh the risks of mercury exposure. By this logic, pregnant women should be encouraged to eat any fish high in omega-3s, even if that same fish is also high in mercury.

In the new guidelines, fish are grouped into "best choices," "good choices," and "fish to avoid," based on their mercury levels. And the agencies set limits for the amount of fish to eat from the best and good choices groups, advising two to three 4-ounce servings per week of the best choices or one weekly 4-ounce serving of the good choices. (For children ages 4 to 7, the serving sizes are 2 ounces.)

“Consumer Reports applauds this change,” Halloran says. “We strongly objected to the net-benefits approach and advocated with the FDA for more than five years to abandon it. Rather than essentially being told to eat more fish, we felt that it was important for pregnant women to hear the message to eat fish but to choose those that were lowest in mercury.”

In addition, based on input from consumer groups, including Consumer Reports, the agencies expanded the “fish to avoid” list from four to seven species.

Fish industry groups have criticized these new recommendations, saying they will confuse consumers and lead to less fish consumption and thus fewer healthy pregnancies overall. “FDA numbers show that pregnant women eat less than 2 ounces of fish per week as it is,” says Rima Kleiner, the National Fisheries Institute’s registered dietitian. “The FDA’s clinical goal, originally, was to increase that number. That message is lost in this advice.”

CR's Recommendations

Although Consumer Reports food safety experts agree with the agencies' advice to have two to three fish servings per week, they think it is safer to stick to a more limited list of 17 types of low-mercury fish and avoid very high-mercury fish altogether. Below are CR's recommendations.

Lowest-mercury fish: oysters, salmon (wild and Alaska, canned or fresh), sardines, scallops, shrimp, squid, and tilapia.
• A 132-pound woman can safely eat up to 36 ounces per week; a 44-pound child can safely eat up to 18 ounces.

Low-mercury fish: Atlantic croaker, Atlantic mackerel, catfish, crab, crawfish, flatfish (flounder and sole), haddock, mullet, pollack, and trout.
• A 132-pound woman can safely eat up to 18 ounces per week; a 44-pound child can safely eat up to 6 ounces.

Fish to avoid: bigeye tuna (often used in sushi), Gulf tilefish, king mackerel, marlin, orange roughy, shark, and swordfish.
• These fish are too high in mercury to be safe for women of childbearing age, those who are pregnant or breastfeeding, and young children.

What About Canned Tuna?

Tuna is widely popular. Next to shrimp it’s the type of fish that’s most often consumed in the U.S., so it is important that women understand the benefits and risks of consuming it.

The FDA told Consumer Reports: "It’s important to note that this advice is targeted to certain women and young children. As a result, we took a cautious and highly protective approach in determining which fish belonged in each category, including a peer review of the data, and are comfortable with the limits we are providing." 

However, our experts disagree with the inclusion of canned and some fresh tunas on the FDA’s “best choice” and “good choice” lists. Their analysis of the FDA data revealed that although canned light tuna (which is on the agencies' “best choice” list) has on average only a third of the mercury found in white or albacore tuna, 20 percent of the samples tested contained almost double the average amount of mercury listed by the agency for that species. In other words: On average, mercury levels for canned light tuna are low, but individual cans may contain high amounts of the toxin. And there’s no way for pregnant women—or any consumer—to tell which cans contain the potentially dangerous levels.

The same Consumer Reports analysis found that eating 4.3 ounces—less than one can—of albacore (an FDA “good choice” option) would put a 132-pound woman above the EPA’s safe consumption limit for mercury for a week. A 44-pound child would exceed that limit by eating just 1.5 ounces—about a third of a can.

Bottom Line

It’s possible to reap the full benefits of a diet high in omega-3s while still protecting yourself, your developing baby, or your growing child from the effects of too much mercury. The trick is to choose the right kinds of fish. By picking something that’s so high in mercury that one serving maxes out your weekly fish allotment (i.e., a fish from the agencies’ “good choices” list), you potentially limit your omega-3 consumption.

“Eating from FDA’s 'good choices' list, one 4-ounce serving would be all the fish women of childbearing age, pregnant women, and young children could have that week and stay within the mercury limit,” Halloran explains. “Since eating fish a couple times a week has many benefits, it would seem this is not a good group to choose from.”

“The best option for people in the at-risk groups is to stick to the fish on Consumer Reports' low- and lowest-mercury fish lists,” Halloran says.

Most important, pregnant women should not eat any tuna at all, because even some of the types that end up in cans can be quite high in mercury. This advice extends to sushi made with tuna.

Other women as well as children can have light tuna in moderation. The safe amount depends on the individual’s weight. For example, a 44-pound child could have about 3½ ounces of light tuna per week and a 132-pound woman could have about 11½ ounces. (A typical can of tuna has 5 ounces.) Keep in mind, though, that these recommendations assume an individual does not eat any other fish that week.

To plan your weekly fish diet, see our mercury resource page and use the Got Mercury? calculator.

Editor's Note: This article has been updated to include responses from the Food and Drug Administration.