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ISEE-261 Introduction: Organochlorine (OCs) compounds were widely used as insecticides in the United Statesfrom 1940 through the 1970s, but following recognition of their persistence in the environment, toxicity in animals, and their potential for endocrine disruption, most have been severely restricted in use. Although there is substantial evidence for the foetal toxicity of OCs in animals, there is conflicting information on human reproductive effects. For example, there are, a few studies in humans that suggest that OCs, in particular DDT/DDE, can reduce foetal growth or gestational duration, but results are not consistent. Aim: The purpose of the present analysis was to determine whether decreased fetal growth or shortened length of gestation was associated with maternal serum levels of 11 different OCs: p,p'-DDT, p,p'-DDE, o,p'-DDT, hexachlorobenzene (HCB), β-hexachlorocyclohexane (β-HCCH), gamma-hexachlorocyclohexane (gamma-HCCH), dieldrin (DIE), heptachlor epoxide (HE), oxychlordane (OXY), trans-nonachlor (TNA), and mirex. Methods: We investigated this relationship in the CHAMACOS birth cohort of low-income Latinas living in the Salinas Valley, an agricultural community in California. Serum levels of OCs in 385 pregnant women were assessed in relation to their infants' length of gestation, term birthweight, crown-heel length, head circumference and ponderal index. We also examined associations between maternal serum OC levels and low birth weight, preterm delivery, and small for gestational age, but numbers were small. Results: Median serum concentrations of several OCs (such as DDT, DDE and HCB) were much higher than U.S. averages reported by CDC. However, there were no adverse associations between maternal serum OC levels and birthweight, length, head circumference or ponderal index. Decreases in gestational age were seen with increasing levels of lipid adjusted HCB (log10) (adjusted β=-0.47 weeks, p=“0.05”). We did not find decreases in gestational duration associated with any of the other OCs. Conclusion: Overall, OC exposure was not associated with decrements in measures of foetal growth or length of gestation. However, since the CHAMACOS cohort has a relatively low rate of adverse fetal growth outcomes, we are limited in ruling out effects seen in other studies. Our finding of decreased gestational age related to HCB does not seem to have had clinical implications for this population, given the relatively low rate of preterm delivery (6.5%). Our results add to the growing body of research in humans on the relationship between in utero exposure to OCs and reproductive outcomes.