However, raw fish should also be avoided during pregnancy because it may contain parasites such as flukes or worms. Cooking and freezing are the most effective methods for killing the parasite larvae found in fish. The USFDA recommends cooking the fish to 140°F. The fish should appear opaque and flaky when done. Freezing the fish at - 10°F for at least 168 hours (7 days) will also kill most larvae [6]

Mercury in Fish and Seafood

Fish and seafood can be an important part of a balanced diet for pregnant women and those of childbearing age who may become pregnant. Indeed low consumption of fish was found to be a strong risk factor for preterm delivery and low birth weight in one study [7]. In addition maternal consumption of low levels of seafood may have a detrimental effect on child development [8]. Nonetheless, some large long-lived fish contain high levels of a form of mercury called methyl mercury that may potentially harm an unborn baby's developing nervous system.

Small fish absorb methylmercury from water as they feed on aquatic organisms. The longer the fish lives the more methylmercury the fish accumulates in its body. Large, long-lived, larger fish that feed on other fish (high in the food chain) accumulate the highest levels of methylmercury . In the United States the limit for methyl mercury in commercial marine and freshwater fish is 1.0 parts per million (ppm). In Canada the limit for total mercury content is 0.5 (ppm) [1,2]. Although the mercury levels found in most commonly consumed fish and seafood are well below these limits, the mercury levels found in several predator species frequently exceed 1.0 ppm
 

The FDA advises that pregnant women, women who may become pregnant, nursing mothers, and young children should avoid eating shark, swordfish, king mackerel and tilefish due to high levels of mercury in these fish [1] (Table 1).

The FDA also recommends that pregnant women can safely eat an average of 12 ounces of other types of COOKED fish each week You may eat up to 6 ounces (one average meal) of albacore tuna per week. Emphasis is placed on choosing a variety of  fish low in mercury such as shrimp, canned light tuna, salmon, pollock, and catfish. [1].So, when choosing your two meals of fish and shellfish, you may eat up to 6 ounces (one average meal) of albacore tuna per week.

If no local advisories about the safety of fish caught by family and friends are  available, eat up to 6 ounces (one average meal) per week of fish you catch from local waters, but don't consume any other fish during that week."[1]

Washington State Department of Health [4]
Advises women of childbearing age and children under six to:

• Avoid fresh caught or frozen tuna steaks. (Fresh or frozen tuna normally contains about .32 parts per million of mercury, but that number can go as high as 1.3 parts per million.)
• Limit the amount of canned tuna* they eat, based on their bodyweight.
  • Women of childbearing age should limit the amount of canned tuna they eat to about one can per week (six ounces.) A woman who weighs less than 135 pounds should eat less than one can of tuna per week.

Wisconsin Department of Natural Resources [5]
Advises women of childbearing age to:

• Limit tuna steaks, halibut, orange roughy to one meal per month
• Limit cod, pollock, haddock, tuna (6ounce can) to one meal per week
• Limit salmon and shellfish to 2 to 3 meals per week.

The Environmental Working Group lists other types of fish that may be contaminated by mercury, as well as by PCBs, include bluefish and striped bass, and freshwater fish (such as salmon, pike, trout, walleye) from contaminated lakes and rivers.
Their expanded list is available online: Brain Food: What Women Should Know About Mercury Contamination in Fish (PDF file)

PPM=parts per million
ND=Not detectable  - mercury concentration below detection level (Level of Detection (LOD)=0.01ppm)

TABLE 2. Fish or Shellfish That May at Times Contain High Levels of Mercury [3]
 

SPECIES	MEAN (PPM)	RANGE (PPM)
Bass (Chilean)	0.386	0.085 - 2.180
Mackerel Spanish  (Gulf of Mexico)	0.454	0.070 - 1.560
Halibut	0.252	ND -1.520
Snapper	0.189	ND - 1.366
Lobster Northern (American)	0.31	0.05 - 1.31
Tuna all (Fresh or frozen)	0.383	ND - 1.30
Grouper (All species)	0.465	0.053 - 1.205
Monkfish	0.180	0.020 - 1.020
Bass (Sea bass/ Striped Bass/ Rockfish)	0.219	ND - 0.960
Orange Roughy	0.554	0.296 - 0.855

PPM=parts per million
ND=Not detectable  - mercury concentration below detection level (Level of Detection (LOD)=0.01ppm)