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.