Fish consumption. (Drugs, Pregnancy, and Lactation).

This column traditionally discusses the safety of medicinal drugs used to treat pregnant and lactating women. But every so often practitioners are asked by patients about environmental factors that may affect pregnancy outcome. So from time to time, I hope to introduce a topic of interest in this area.

Fish consumption during pregnancy is clearly such an issue. In general, eating some fish is encouraged for all women, because it is a good source of protein and omega-3 polyunsaturated acids. Essential fatty acids are important for neurologic development.

But inorganic mercury--a neurotoxin in adults, children, and the fetus--pollutes water, air, and soil through natural deposits and can contaminate food and fish, In the late 1960s, the world learned about the congenital effects of mercury poisoning with the focus on children affected by the industry deposits of mercury in Minamata Bay in Japan. The babies, whose mothers ate fish from the bay while pregnant, had brain damage and severe cerebral palsy because of exposure to high levels of methylmercury in their mother's blood.

Methylmercury, formed by the interaction between bacteria and mercury in the water, is the most common organic mercury and is of interest because it can contaminate seafood, building up in the tissue of fish.

In 1972 the World Health Organization stated that pregnant women should have levels that were no more than 5 ppm in the hair to avoid potential risk to the fetus. They also said that women and men should consume no more than 0.47 [micro]g per kg of body weight of mercury per day. In 1996, the Environmental Protection Agency set a new reference dose: 0.1 [micro]g/kg per day.

Data from other studies are generally reassuring because they are based in populations that eat mostly fish. For example, several studies in the Seychelles islands found that children at 66 months had no neurodevelopmental deficits, even though their mothers had mean mercury hair concentrations of 6.8 ppm, exceeding the U.S. lower limit of 5 ppm.

Another study took place in the Faeroe Islands where inhabitants eat mostly pilot whale meat, which contains 10 times higher mercury levels than the smaller fish consumed in the Seychelles. Researchers in that study found no increased rates of clinical or neurophysiologic abnormalities in the more than 900 children who were studied at age 7. They did find subtle reductions in some neuropsychologic tests, which were linked to ...