Estimates of the ratio of cord to maternal blood mercury are used to model fetal exposure based on maternal blood levels and to derive the maternal reference dose. There is a persisting need for data on the variability of this ratio (Rice et al. 2003
). Our mean ratio (2.18 ± 1.72) is consistent with mean values for 10 studies summarized in a recent review (Stern and Smith 2003
), which ranged from 1.09 to 2.32. Higher cord blood mercury may be due to greater binding of mercury by fetal hemoglobin, higher fetal hemoglobin concentrations, and continued fetal exposure from repeated swallowing of urine in amniotic fluid and from intestinal resorption of mercury from fetal feces. Maternal mercury is lost in urine and feces.
We were unable to demonstrate significant increases in mercury levels because of residential or work-site proximity to the WTC site in the weeks after 11 September. Although mercury may have been released during the WTC tragedy, effects of that exposure may have been obscured by the short half-life of metallic mercury in blood, the collection of blood months after the event, our lack of inorganic mercury measures, the high and variable contribution of methylmercury from seafood, and possibly mercury from earlier environmental exposures, especially among our China-born women, whose levels were very high. Nevertheless, the relation of blood total mercury to later child development does not depend on the source of blood mercury.
We found no significant relationship of total mercury in cord or maternal blood to newborn size or gestational age at birth. In contrast, the relation to child development was clear and increased between the ages of 1 and 3 years. The (log) mercury level in cord blood was associated with a significant, –4.2 point decrement in the 36-month PDI of the BSID-II and with decrements of –3.4, –2.9, and –3.8 points in Performance, Verbal, and Full IQ, respectively, on the WPPSI-R at 48 months, in regression models that controlled for selected determinants/confounders. Several previous studies reporting adverse effects of mercury on cognitive development have studied populations with higher levels of mercury than in our cohort, largely from methylmercury (e.g., Grandjean et al. 1997
: cord total mercury geometric mean = 22.9 μg/L in Faroe Island children; present study = 4.4 μg/L).
In the present study, maternal blood mercury was similar to values from the 1999–2002 National Health and Nutrition Examination Survey (Jones et al. 2004
), where the geometric mean for women of child-bearing age was 0.92 μg/L (present study, 0.91 μg/L), with 5.66% (present study, 5.95%) having levels ≥ 5.8 μg/L. Our median cord blood total mercury level was 4.3 μg/L (32.1% ≥ 5.8 μg/L), compared with 0.85 μg/L in a recent Polish study (Jedrychowski et al. 2006
). In a Swedish study (Bjornberg et al. 2003
), median cord blood methylmercury was 1.3 μg/L (range, 0.1–5.7 μg/L), with inorganic mercury about one-tenth as high (0.15 μg/L; range, 0.03–0.53 μg/L). Despite the difference in levels, all three studies reported a positive association between blood mercury levels and fish consumption, whether measured as number of types of fish/seafood consumed during pregnancy (present study), maternal fish/seafood consumption in the year preceding pregnancy (Bjornberg et al. 2003
), or estimated weekly fish consumption during pregnancy (Jedrychowski et al. 2006
We observed higher mercury levels in China-born Asians than in the other ethnic groups. Higher hair mercury levels, even after controlling for fish/seafood consumption, have been reported previously in Asians compared with non-Asians recruited from across the United States (Patch et al. 2005
). Differences in frequency of fish/seafood meals or in the amount consumed at each such meal could be partly responsible for differences in blood mercury levels in our ethnic groups, because we used only the number of different types of fish/seafood eaten during pregnancy to classify women; however, even among those not eating seafood, China-born Asians had higher mercury levels than did other groups.
The cord:maternal mercury ratio was significantly higher for fish eaters than for non-fish eaters, whereas it was not higher for China-born than for non-China-born Asian women. These results support the belief that the high mercury levels of China-born Asians were at least partly related to factors other than seafood consumption, such as from pollution in China from mercury-containing coal used for energy production (Pottinger et al. 2004
) or from use of herbal drugs that contain mercury (California Poison Action Line 2002
It is noteworthy that, in reduced models with mercury level controlled, maternal fish/seafood consumption during pregnancy was associated with an 8.7-point increase in PDI score at 36 months and a 5.6 point increase in Verbal and Full IQ scores at 48 months, suggesting the value of consuming fish/seafood during pregnancy. Recent work reanalyzing data collected in the 1980s in the Faroe Islands (Budtz-Jorgensen et al. 2007
) has demonstrated the importance of considering the positive effects of maternal fish intake on child development at 7 and 14 years of age, when estimating the adverse effects of high methylmercury exposure. In that study, fish intake had the greatest positive effect on motor function outcomes, consistent with our observation of a positive relation of fish/seafood consumption to the child’s PDI score at age 3 years of age. A beneficial effect of fish consumption on BSID-II cognitive and psychomotor scores at 1 year of age, with a measurable negative effect of (low) blood mercury levels, has also been reported (Jedrychowski et al. 2006
), as has a positive relation of fish consumption to a Visual Recognition Memory score at 6 months of age, with a negative relation to (low levels of) hair mercury (Oken et al. 2005
These studies, combined with ours, show positive developmental effects of maternal fish/seafood consumption and negative effects of blood mercury at six different ages (0.5, 1, 3, 4, 7, and 14 years) over a very wide range of mercury levels. Apparently, maternal consumption of fish/seafood can raise fetal mercury levels yet enhance cognitive development in the child, probably because of other constituents, particularly omega-3 fatty acids (Innis 2004
). Thus, inclusion of a measure of maternal fish/seafood intake appears to be important for demonstrating the true magnitude of the detrimental effect of low levels of mercury on development.