Maternal prenatal blood mercury is not adversely associated with offspring IQ at 8 years provided the mother eats...

Conflicting evidence concerning possible harm from mercury (Hg) in regard to offspring cognition if the woman eats fish has prompted this study to examine evidence from a British pre-birth cohort to investigate the relationship between the two.

Pregnant women (median prenatal blood mercury 1.86microg/L) resident in the study area with delivery between April 1991 and December 1992 were followed up and verbal, performance and total intelligence quotient (IQ) of 2062 offspring were measured at age 8. Analysis treated IQ as (a) continuous and (b) the lowest 25% of the distribution. Multiple and logistic regression analyses took account of social and demographic variables. Stratification considered children of fish eaters separately.

Before adjustment, mean full-scale IQ increased with increasing Hg (change with 1SD of Hg=+2.02; 95%CI+1.40,+2.64 IQ points; P <0.0001); after adjustment effect size was reduced although still positive (+0.61;95%CI -0.06,+1.29 IQ points; P=0.073). The adjusted positive relationship was stronger when fish-eating mothers were considered separately (+0.84:95%CI +0.13,+1.56 IQ points; P=0.021) in comparison with the outcomes for non-fish eaters, where the adjusted relationship was negative (-2.22;95%CI -5.00,+0.56 IQ points; P=0.117). The binary outcome showed a similar pattern with the adjusted OR for non-fish-eaters 1.79 (95%CI 1.10,2.93; P=0.019) per SD of Hg, significantly different from that for fish consumers (0.94;95%CI:0.82,1.08)(P_interaction<0.05). There were no differences between the sexes in the associations, nor did the level of the mother's blood selenium change the effect sizes.

The relationship between intrauterine exposure to mercury and offspring IQ appears to be benign provided the mother consumes fish.