Research has shown that pregnant women should limit their intake of swordfish, tuna, shark and king mackerel because of their high levels of mercury, which can cause brain damage in unborn children.

The accumulation of toxic mercury, methyl mercury, found in fish and seafood is caused by pollution that in excess levels has been found to be particularly toxic to the nervous system and the developing brain of children. Traces of methyl mercury were found to be the highest in longer-lived large fish

Health officials have recommended that women of childbearing age should select fish such as shrimp, salmon, pollock and catfish.

The suggested guidelines for eating fish included 12 ounces per week of fish low in mercury and no more than six ounces of white tuna or fish caught by family or friends.

According to research, an average of 630,000 U.S. children are born each year with toxic amounts of mercury in their blood.

Studies also revealed that white tuna contained three times as much mercury as light tuna. For this reason, experts have warned women of childbearing age to limit their consumption of albacore white tuna.

Despite the mercury concerns, the U.S. Tuna Foundation has described tuna as "nature’s healthy fast food" that is low fat and contains many nutrients.

Yahoo! News March 18 & March 24, 2004