Prenatal exposure to methylmercury and child development: influence of social factors

In the Seychelles Child Development Main Study (SCDS), at 19 months of age, enhanced MDI scores were associated with increasing methylmercury (MeHg) exposure in higher caregiver IQ groups at several levels of family income. We performed a similar analysis of the 66-month evaluations to determine if the modifying influences of social and environmental factors were consistent with those previously observed.

Prenatal MeHg exposure was determined by analysis of maternal hair growing during pregnancy. Children in the cohort (N=711) were evaluated for cognitive ability (McCarthy Scales of Children's Abilities), language development (Preschool Language Scale), drawing and copying (Bender Gestalt Test), scholastic achievement (the Woodcock-Johnson Test of Achievement), and behavior (the Child Behavior Checklist). Interactions between prenatal MeHg exposure and caregiver intelligence, socioeconomic status (SES), home environment, and gender were examined by multiple regression analysis.

The median prenatal MeHg exposure was 5.9 ppm (range=0.5-26.7 ppm). Gender significantly influenced the association between prenatal exposure and drawing and copying; however, the effects were not consistent. Prenatal exposure interacted with one or more social or environmental covariates for general cognitive ability, overall language ability, and prearithmetic achievement. Again, the effects were not consistent across either endpoints or covariate categories.

Evidence of a small influence by social and environmental variables at 66 months is neither internally consistent nor consistent with earlier results. Overall, a consistent pattern of effect modification (EM) has not been observed, suggesting that the results may be due to chance.