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Manganese is both an essential element and a known neurotoxicant to children. High manganese exposures have been associated with negative reproductive outcomes in animals, but few epidemiologic studies have examined the effects of human fetal manganese exposure.We studied the association between maternal and umbilical cord blood manganese levels and birth weight in a cohort of 470 mother-infant pairs born at term (≥37 weeks gestation) in Ottawa County, Oklahoma. Nonlinear spline and quadratic regression models were used to test the hypothesis of an inverted U-shaped relationship between manganese levels and birth weight.Mean (standard deviation) concentration of manganese was 2.4 (0.95) μg/dL in the maternal blood and 4.2 (1.6) μg/dL in the cord blood. Umbilical cord manganese was not associated with birth weight. A nonlinear relationship was observed between maternal manganese and birth weight after adjusting for potential confounders. Birth weight increased with manganese levels up to 3.1 μg/L, and then a slight reduction in weight was observed at higher levels. Compared with the 3.1-μg/L point of inflection, birth weight estimates at the 5th (1.3 μg/L) and 95th (4.0 μg/L) percentiles of exposure were −160 g (95% confidence interval = −286 to −33) and −46 g (−38 to 131), respectively.Maternal blood manganese levels during pregnancy are associated with birth weight in a nonlinear pattern in full-term infants. These findings suggest that manganese may affect fetal growth. Possible detrimental effects of elevated manganese levels on the fetus should be further examined in more highly exposed populations.