When Jennifer Power was pregnant with her first child, she forced herself to eat fish. She doesn’t really like the food, but she thought it would be good for her baby. “My OB/GYN encouraged me to eat fish for the omega-3 fats,” she says. But Power also worried about the risk of mercury, and wondered how to balance the health benefits with the real risk of mercury consumption.
It turns out Power was right to focus on fish. In July 2019, the US Food & Drug Administration released new guidelines that recommend pregnant women consume eight to 12 ounces of low mercury fish per week to get essential nutrients for pregnancy, particularly the omega-3 fat called docosahexaenoic acid (DHA). This is more than what’s recommended in Health Canada’s 2007 guidelines, which recommend five ounces of low-mercury fish each week.
Bruce Holub, professor emeritus in the department of Human Health and Nutritional Sciences at the University of Guelph, says that recent studies indicate that pregnant women may benefit from more DHA than previously thought, so the Health Canada guidelines may be outdated. "Two major international clinical trials found that supplementation with 500 or 800 mgs of DHA per day during pregnancy reduced premature births by at least 50 percent along with a dramatic reduction in the need for admission into intensive care units,” says Holub.
Holub adds that getting enough DHA also reduces the risk of low birthweight babies. Plus, since DHA passes through the placenta, it also yields long-term neurological benefits for the infant. “DHA supports brain and eye development of the infant both during pregnancy and after birth,” he says.
Kristin Brown, a registered dietitian specializing in maternal health in Fredericton, New Brunswick, adds that fish is also a great source of vitamin D, selenium, iodine and zinc, which are essential nutrients for a healthy pregnancy. A four-ounce serving of fish has about 25 grams of protein, which contributes to the 75 to 100 grams of protein that’s required daily to support normal fetal growth.
But Brown says that some of her pregnant clients avoid fish entirely because they are concerned about mercury, a hazardous metal that can impair a baby’s brain, nervous system, vision and motor skills.
Fish accumulate mercury from contaminated waterways, and levels of mercury usually increase with fish size, age and diet. So a large fish that eats smaller fish and has a long lifespan will accumulate the most mercury. The mercury in waterways gets converted to highly-toxic methylmercury, which is absorbed into the body more easily than inorganic mercury, and does cross the blood-brain and placental barriers. That means it can affect a baby’s early brain development if your mercury intake is at very high levels (i.e. you often eat the fish listed below).
Pregnant women should avoid large, predatory, high-mercury fish like marlin, orange roughy, shark, king mackerel, swordfish, tilefish and some types of tuna. You also need to check fish advisories for mercury levels before consuming anything you’ve caught in local lakes.
You should, however, eat lots of low-mercury, high omega-3 fat fish, like salmon, trout, herring, mackerel, sardines and char. Other low-methylmercury choices include cod, haddock, pickerel, tilapia, shrimp, lobster and scallops, but these are not as high in omega-3 fats. Whether you buy wild or farmed fish is a personal choice based on cost, availability and environmental sustainability, since mercury can potentially be found in both.
For Power, her OB/GYN’s recommendation to eat fish inspired new habits. “I actually came to love the ease and convenience of canned salmon, and was a full-fledged fish eater when I was pregnant with my second child three years later,” she says. “And because I eat fish, I think it has inspired my kids to eat fish too.”
Larger species of tuna, such as albacore (“white”) and bigeye, accumulate more methylmercury than smaller species, such as skipjack, tongol or yellowfin (“light”) tuna, which are lower in mercury and safer to eat during pregnancy.
But while imported albacore tuna is high in methylmercury, Canadian North Pacific albacore tuna is not, according to the Canadian Food Inspection Agency.
“Look for white albacore tuna that’s labelled ‘Product of Canada,’” advises Toronto dietitian Rosie Schwartz. “They are smaller and have lower mercury levels, so pregnant women don’t need to limit their intake.” Regular grocery store albacore tuna is not usually Canadian. Schwartz suggests looking for canned or frozen Canadian albacore tuna at health foods stores, specialty food stores, or buying it online. If you order a tuna sandwich in a restaurant, assume it’s imported albacore, unless otherwise stated.