Eating fish during pregnancy: What's the right approach?

Shutterstock.com

Figuring out the right amount, and types, of fish to eat during pregnancy can feel like yet another tricky decision meant to test a conscientious mom-to-be. Much ado has been made of the omega-3 fatty acids in fish that appear to promote healthy development of the fetal brain. Research that gets a lot of attention in certain corners of the Web suggests that kids born to mothers who had more fish-derived omega-3s during pregnancy fare better than the kids whose moms didn't. The reported benefits associated with omega-3s include fewer behavior problems, better verbal skills, and even higher IQs. So it's hardly a shock that the meticulous mommy set has taken an interest in seafood.

At the same time, many expectant mothers are perplexed by the toxins conundrum. Mercury, which can harm the fetus's developing nervous system, is found in at least trace amounts in nearly all seafood and at far greater levels in certain fish.

So which fish should a pregnant woman eat? U.S. News discussed the dilemma with two experts: Gideon Koren, physician in the department of clinical pharmacology and toxicology and director of the Motherisk program (dedicated to counseling pregnant women about risks related to drugs, chemicals, disease, nutrition, and environmental agents) at the Hospital for Sick Children in Toronto; and Emily Oken, a physician and assistant professor in the department of ambulatory care and prevention at Harvard Medical School who has researched the risks and benefits of fish consumption during pregnancy and its effects on child development. Here's what we learned:

Pick the right fish. Not all are the same.

Big fish that are high on the food chain tend to have greater mercury levels than smaller fish, so size does matter. And not all seafood is equally plentiful in the coveted omega-3s. For example, shrimp and pollack are low in mercury but don't offer a lot of omega-3 compared with other seafood, says Oken. Other fish are high in both omega-3s and mercury.

Generally speaking, salmon (whether wild or farm raised) tends to be low in mercury and rich in omega-3s, as do herring, trout, and sardines. Canned tuna, Oken notes, usually has moderate levels of mercury. The fish found to have the highest mercury levels, says Koren, are fresh or frozen tuna, shark, swordfish, escolar, marlin, and orange roughy. Pregnant women are advised to limit these to twice per month, he says. The Food and Drug Administration and the Environmental Protection Agency also warn against king mackerel and tilefish.

Balancing omega-3s and mercury is complicated.

The FDA and the EPA recommend that pregnant women eat up to 12 ounces, or about two servings, of low-mercury fish and shellfish per week. Following that recommendation can be a good way to get omega-3s without getting too much mercury--but it's no guarantee. On some measures of behavior and brain development, Oken says, the children of "women who have two to three servings of supermarket fish [per week] tend to do better" than those who eat less fish. But limiting fish intake by number of servings is a crude way to restrict mercury intake, Oken says, because most people aren't aware of which types of fish are high in mercury.

"Even different types of fish caught in the same lake can have different levels of mercury," says Koren. If you eat locally caught fish, you may need to inquire with the health department to determine the levels of mercury in a given type of fish or shellfish and to learn about special seafood advisories the department may have issued.

In fish and fish oil supplements, DHA and EPA are, generally speaking, the types of omega-3s thought to yield the greatest benefits, and protection of cardiovascular health is the most thoroughly researched of their benefits. In fetal brain development, though, most research points to DHA as the prime actor. EPA, however, may offer fetuses other benefits, such as protection against asthma, says Oken.

Nevertheless, experts caution that the research supporting the recommended level of fish consumption is not definitive. "At the present time," Koren says, "the hard evidence that we demand--to say what to do--is not there." He notes that a randomized study designed to confirm a link between maternal omega-3 intake and higher offspring IQ found no significant effect.

Worried you've consumed too much mercury? A test is available.

Getting a hair test is the most reliable way to determine a woman's level of mercury and, thus, the amount of neurotoxin exposure of her fetus, Koren says. In expectant mothers, mercury levels greater than 0.3 micrograms of mercury per gram of hair can have negative effects on the fetus. For women who've eaten a lot of fish during or before pregnancy, he says, "a hair test can put their mind at ease." But he doesn't recommend it for most moms-to-be. For people who avoid the source of exposure, high mercury levels will go down over time, he says.

Supplements are not a must-do but typically aren't harmful.

Again, there is no definitive rule here. "Is taking a 200-to-300-mg DHA supplement per day harmful? Almost certainly not," says Oken. It's not necessarily worth the cost, though, she says. And if you're considering a fish oil supplement, Oken warns specifically against fish liver oils, which tend to contain contaminants.

Koren agrees that supplements are generally safe, and he isn't opposed to pregnant women taking them. But he warns women not to worry about the cognitive health of their babies if they choose not to take omega-3s. "We should be very careful before we claim something is effective in pregnancy," he says, "because it may be construed that if you do not do it, you will negatively affect your baby." Industry influence--from fishmongers to supplement purveyors--should not be dismissed, either, he notes. Until definitive science is available, women simply need to make informed decisions they're comfortable with.

(c) 2009 U.S. News & World Report