What fish should I avoid while pregnant?

DEAR DOCTOR K:

I recently found out that I’m pregnant. I’d like to continue eating fish, but I understand some fish contain mercury, which could be harmful to my baby. What fish should I avoid?

DEAR READER:

Fish are a great source of lean protein, and many types are rich in heart-healthy omega-3 fatty acids, which help brain and nerve development and protect the heart. In fact, current dietary guidelines recommend that women who are pregnant eat 12 ounces of seafood a week.

But as you noted, some species of fish do contain worrisome amounts of methylmercury. This toxin is especially dangerous to developing brains. High-mercury fish you should avoid during pregnancy include swordfish, shark, king mackerel and tilefish.

A recent study may add two more fish to that list. The study hints that salmon and tuna may also boost mercury levels.

The new study looked at what types of seafood Americans eat and how often, and how that affects mercury levels in the blood. The vast majority of participants had blood levels of mercury in the safe zone. Not surprisingly, the more fish people ate, the higher the levels of mercury in their blood. Those who consumed swordfish, shark and other high-mercury fish were the most likely to have higher blood levels of mercury. But some study participants who reported that they ate only salmon or tuna also had high mercury levels.

My Harvard Medical School colleague Dr. Emily Oken has studied women’s fish consumption during pregnancy. She explains that it’s complicated to tease out the harmful effects of mercury. That’s because the primary source of mercury in our diets is fish. And fish have nutrients that are beneficial to the brain and the heart — the same organs that mercury may harm.

Low-mercury fish options include trout, herring, pollock, flounder and sole. Here is a table listing additional low-mercury seafood choices:

Low-mercury seafood choices

There are many different ways to get the recommended weekly amount of omega-3 fatty acids. You can do it with a single meal of fish high in omega-3 fatty acids, or multiple meals of species with lesser amounts. (An expanded version of this table is available in the online Nutrition Journal.)

One 6-oz serving per week of OR two 6-oz servings per week of OR three 6-oz servings per week of
salmon (farm raised, wild caught, or canned)
whitefish/walleye
herring
anchovies (canned)
trout (farm raised)
Atlantic mackerel
sardines (canned)
trout (wild caught)
mussels
pollock
salt cod/bacalao
squid/calamari
ocean perch
flat fish
flounder
sole

In this column I try to make things simple, but the reality is that things are rarely as simple as we would like. Eating fish is a good example. Like all foods, fish contain many different substances. Many of them are healthy, but some of them are unhealthy.

So what do you do? As with so many things in life, the key is moderation. Continue to enjoy fish, just not too much. Since you’re pregnant, eat 12 ounces of fish a week. And choose the fish that contain relatively low levels of mercury. Stay away from high-mercury species.

Finally, remember that “moderation” also applies to advice. If you’ve already had a meal or two of high-mercury fish during your pregnancy, please don’t worry that you’ve injured your baby’s brain. A meal or two of high-mercury fish is not like taking poison.