Among the types of fish you can buy in the typical grocery store, farm-grown tilapia contains the least amount of mercury and thus is the safest for pregnant women to eat.

But does this mean other popular fish are less safe? Not unless you are planning to give birth or eat a diet of nothing but tilefish and shark. In fact, the February Harvard Health Letter suggested that for most adults, the risk of ingesting mercury is outweighed by the benefits of eating a diet high in omega-3s.

The Food and Drug Administration doesn't evaluate the safety of every type of fish caught commercially and doesn't consider the safety of fish caught by weekend anglers at all. But it does track mercury in fish through two separate programs and has found over the years that although most fish that contain mercury are safe to eat, a consumer's safest bets are tilapia and salmon; mercury typically is undetectable in either fish.

"Generally, mercury levels in fish are very low and we don't see a lot of fluctuation on the lists over time," says David Acheson, M.D., chief medical officer of the FDA's Center for Food Safety and Applied Nutrition.

However, in March 2002, the FDA published an advisory suggesting that women who are pregnant or who are planning to become pregnant shouldn't eat a lot of shark, swordfish, king mackerel or tilefish. These larger fish typically have higher mercury levels than do the small tuna that are usually canned, or wild coho salmon and farmed catfish.

Women planning to become pregnant were warned of the mercury issue because it takes about a year to flush the metal from the human body. A fetus' developing nervous system can be affected by mercury.

Natural Foods Merchandiser volume XXIV/number 3/p. 22