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Am Fam Physician. 2006;74(9):1616-1619

The heavy metal mercury is found throughout the environment; however, it is more concentrated in animal tissue. Human exposure to mercury primarily is through consumption of fish, particularly tilefish, swordfish, king mackerel, and shark. Mercury exposure is problematic in pregnant women because of the neurotoxic effects in utero; therefore, seafood consumption during pregnancy should be limited. The current recommendation is to consume only 12 oz per week of species with low concentrations of mercury or only 6 oz per week if the species is unknown (see accompanying table). Despite these recommendations, 8 percent of women in one study still had higher levels of mercury than recommended. To further assess gestational mercury exposure, Sato and colleagues evaluated fish consumption during pregnancy and how levels of mercury in newborn cord blood were affected.

Avoid entirely
King mackerel, shark, swordfish, tilefish
Six oz or less
Albacore (white tuna), species unknown
12 oz or less
Canned light tuna, catfish, pollack, salmon, shrimp

The women in the study presented to a labor and delivery unit in Honolulu, Hawaii, between July 2004 and March 2005. Inclusion criteria were women 18 to 45 years of age who were in labor with a live fetus, had a gestational age of 36 weeks or more, and were not a cord blood donor. Participants completed a questionnaire about the frequency, amount, and types of fish consumed during pregnancy. Cord blood was obtained after delivery and sent to a single laboratory for heavy metal concentration analyses.

A total of 275 women participated in the study with an average age of 26.2 years, gravidity of 3, parity of 1.5, and gestational age of 39 weeks. The mean level of cord blood mercury was 4.82 mcg per L (24.0 nmol per L). The Environmental Protection Agency’s recommended reference maximum is 5.80 mcg per L (29.0 nmol per L); approximately 28 percent of cord blood samples had mercury levels higher than the recommendation. Overall, 17.6 percent of women exceeded the recommended consumption of one serving of fish per week. Higher consumption of fish correlated with higher levels of mercury in cord blood. Specifically, there was a significant correlation between cord blood mercury levels and the amount of fish consumed during the last month of pregnancy.

The authors conclude that pregnant women who consume high levels of fish, such as those who live in an island state, are three times more likely to have elevated mercury levels in cord blood samples compared with the national average. Educational efforts need to be made to ensure that pregnant women do not consume more than the recommended amount of fish.

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