Prevalence of Exposure to Environmental Metal Mixtures Among Pregnant Women in the United States National Health and Nutrition Examination Survey (NHANES) 1999–2018

Although exposure to metals remains a public health concern, few studies have examined exposure to combinations of metals. This study characterized prevalent combinations of cadmium (Cd), mercury (Hg), and lead (Pb) in women (n = 10,152; aged 20-44 years) who participated in the U.S. National Health and Nutrition Examination Survey (NHANES) 1999-2018. To explore relative metal exposures within this population, Cd, Hg, and Pb blood levels were dichotomized as "high" and "low" categories using median values to represent the center of the metal concentrations in the study population, not thresholds for adverse health effects. The prevalence of the three metal combinations at "high" levels (singular, binary, tertiary combinations) was calculated. Multinomial logistic regression was used to calculate odds ratios for each combination relative to none of these combinations after adjusting for potential confounders. Among the pregnant women (n = 1297), singular Hg was most prevalent (19.2% [95% CI 15.0-23.3]), followed by singular Cd (14.7% [95% CI 11.2-18.2]), tertiary combination Cd/Hg/Pb (11.0% [95% CI 8.7-13.2]), binary combinations Cd/Pb (9.8% [95% CI 7.4-12.2]), Hg/Pb (9.2% [95% CI 6.5-11.8]), Cd/Hg (7.8% [95% CI 6.0-9.6]), and singular Pb (5.5% [95% CI 4.1-6.9]). We found significantly lower odds of having Cd/Hg/Pb (adjusted odds ratio (adjOR) = 0.49: <i>p</i> < 0.001) and Cd/Pb (adjOR = 0.68: <i>p</i> < 0.0364) combinations among pregnant women compared to non-pregnant women. The odds of having higher levels of singular Pb were significantly lower (adjOR = 0.31: <i>p</i> < 0.0001) in women pregnant in their first and second trimesters (n = 563) than in non-pregnant women (n = 6412), whereas, though nonsignificant, the odds were higher for women pregnant in their third trimester (n = 366) (adjOR = 1.25: <i>p</i> = 0.4715). These results indicate the possibility that the fetus might be exposed to higher levels of the metal mixtures due to placental transfer, particularly to Pb, during the early stages of pregnancy. Further research is warranted to understand the relationship between metal combination exposures during pregnancy and maternal and infant health.