Because of the mercury that is present in some fish, the U.S. Food and Drug Administration (FDA) advises the following for women who are pregnant or might become pregnant, women who are nursing, and young children:
Do not eat shark, swordfish, king mackerel, or tilefish. Eat up to two average meals a week of a variety of fish and shellfish. Good choices are farm-raised catfish, salmon, and tilapia; canned light tuna; wild-caught trout, cod, flounder, pollock, salmon, ocean perch, and haddock; and shrimp, scallops, oysters, and clams.
Polychlorinated biphenyls (PCBs) are contaminants that tend to collect in the fatty portions of fish and in the green gland (mustard) of crabs. To reduce fat, and therefore reduce the PCBs, be sure to remove the belly flap, skin, and dark meat before eating. Broil, grill, or bake the fish on a rack so the fat can drip away. Avoid using batter or breading because they tend to hold fat drippings in the fish. Avoid the mustard in crabs.