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]

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)
Orange Roughy	0.571	0.265 - 1.120
Marlin	0.485	0.100 - 0.920
Mackerel Spanish (Gulf of Mexico)	0.454	0.07 - 1.56
Grouper (All species)	0.448	0.006 - 1.205
Tuna all (Fresh or frozen)	0.391	0 - 1.816
Sablefish	0.361	0.090-1.052
Bass (Chilean)	0.354	ND - 2.180
Tuna (canned, Albacore)	0.350	ND - 0.853
Halibut	0.241	ND -1.520
Monkfish	0.181	0.106 - 0.289
Lobster Northern (Species unknown)	0.166	ND - 0.451
Snapper	0.166	ND - 1.366
Bass (Sea bass/ Striped Bass/ Rockfish)	0.152	ND - 0.960
Perch (freshwater)	0.150	ND - 0.325

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