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In developed countries, polychlorinated biphenyls (PCBs) are ubiquitous contaminants of the environment, including foods. Within the range of the resulting low-level exposure, associations of PCBs with lower birth weight have been observed in several studies.To examine further the association of PCBs with birth outcomes, we measured serum levels in 1034 pregnant women who were enrolled in the U.S. Collaborative Perinatal Project in 1959 to 1965 before PCB manufacturing was banned.The multivariate-adjusted odds ratio for preterm birth among those with PCB levels of ≥4 μg/L of total PCBs, compared with those with <2 μg/L, was 1.1 (95% confidence interval = 0.6–2.2); for the same exposure contrast, the odds ratio for delivering an infant who was small-for-gestational-age at birth was 1.6 (0.7–3.7). Birth weight and length of gestation were essentially unrelated to PCB level.In these data, maternal levels of PCBs during pregnancy were essentially unrelated to preterm birth, birth weight, or length of gestation. An association of PCBs with small-for-gestational-age birth was observed, but the results were inconclusive and occurred in the absence of an overall decrease in birth weight.