Fish and seafood can be an important part of a balanced diet for pregnant women and those of childbearing age who may become pregnant. However, some large long-lived fish contain high levels of a form of mercury called methyl mercury that may 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. Emphasis is placed on choosing a variety of other fish such as shellfish, canned fish, smaller ocean fish or farm-raised fish [1].

Washington State Department of Health [4]

Advises women of who are or may become pregnant, nursing mothers and young children to: 

Do not eat - shark, swordfish, tilefish, king mackerel, or tuna steaks.
For Healthy Fish Choices that are low in mercury and other contaminants and high in health benefits see the Healthy Fish Guide  http://www.doh.wa.gov/ehp/oehas/fish/fishchart.htm

Additional information about canned tuna:
Canned Chunk Light Tuna

• Women of childbearing age should limit the amount of canned chunk light tuna they eat to two cans per week. The amount you should eat is based on your body weight see our Meal Size by Weight Table.  New link: http://www.doh.wa.gov/ehp/images/f-mealsize.pdf
• Children under six can eat one half can (three ounce) serving of chunk light tuna per week, plus another fish meal low in mercury that week such as; catfish, cod, flounder, pollock, salmon, shrimp, and trout. A meal size for a child under six can range from one ounce for a twenty pound child, to three ounces for a child weighing sixty pounds.

Canned White (Albacore) Tuna

• Women of childbearing age should limit the amount of canned white (albacore) tuna to one can per week. The amount you should eat is based on your body weight see our Meal Size by Weight Table. New link: http://www.doh.wa.gov/ehp/images/f-mealsize.pdf
• Children under six should eat less than one half a can (three ounces) of canned white (albacore) tuna per week. If your child has eaten canned white (albacore) tuna, your child should not eat any other fish that week. A meal size for a child under six can range from one ounce for a twenty pound child, to three ounces for a child weighing sixty pounds.

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.

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

SPECIES	MEAN (PPM)	RANGE (PPM)
Grouper (Mycteroperca)	0.43	0.05-1.35
Tuna (fresh or frozen)	0.32	ND-1.30
Lobster Northern (American)	0.31	0.05-1.31
*Red Snapper	0.60	0.07-1.46
*Trout Freshwater	0.42	1.22 (max)
*Trout Seawater	0.27	ND-1.19

PPM=parts per million
ND=Not detectable
*Based on limited sample sizes and therefore have a much greater degree of uncertainty

TABLE 3. Fish and Shellfish With Much Lower Levels of Mercury[3]

SPECIES	MEAN (PPM)	RANGE (PPM)
Halibut	0.23	0.02-0.63
Sablefish	0.22	ND-0.70
Pollock	0.20	ND-0.78
Tuna (canned)	0.17	ND-0.75
Crab Blue	0.17	0.02-0.50
Crab Dungeness	0.18	0.02-0.48
Crab Tanner	0.15	ND-0.38
Crab King	0.09	0.02-0.24
Scallop	0.05	ND-0.22
Catfish	0.07	ND-0.31
Salmon (fresh, frozen or canned)	ND	ND-0.18
Oysters	ND	ND-0.25
Shrimps	ND	ND

PPM=parts per million

REFERENCES
1. Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration An Important Message for Pregnant Women and Women of Childbearing Age Who May Become Pregnant About the Risks of Mercury in Fish. Center for Food Safety and Applied Nutrition, U.S. Food and Drug Administration
Accessed:10/4//02