Rauch et al. toxicity. Similarly, we were unable to pinpoint the dominant sources of exposure to OP pesti- cides. Diet and home pesticide use have been identified as important routes of exposure in nonagricultural populations, although Lu et al. (2008) found that switching children from conventional to organic diets for several days reduced urinary OP metabolites to levels near or below the limit of detection, suggesting that diet was the primary source of exposure in that study population. Even if we were able to iso- late a particular pesticide, it would be difficult to connect it to a particular route of exposure, as many different pesticides are used in agricul- ture, as well as in and around the home. The reliance on self-reported dates of last menstrual period to determine gestational age represents another limitation (Lynch and Zhang 2007). This method is imprecise; how- ever, there is no reason to suspect differen- tial misclassification of gestational age with respect to OP exposure. In addition, it was recorded more consistently (97% of mothers) than was ultrasound (8%). Finally, informa- tion on PON enzyme activity or maternal PON1 status might shed more light on PON- mediated susceptibility than infant PON1 genotypes alone. Other strengths of this study are that we presented data from a racially and demographically mixed birth cohort in a sam- ple, intentionally enriched with black women, from the Cincinnati metropolitan area. We also controlled for a wide range of confound- ers, including continuous measures of lead and tobacco exposure, and pregnancy medical complications, and our findings were robust to several sensitivity analyses. Conclusion Prenatal OP insecticide exposure was associated with shorter gestation and lower birth weight in this cohort. The magnitude of these associa- tions differed according to maternal race and infant PON1 genotype status. Although the prior literature on gestational OP exposure and birth outcomes is inconclusive, several studies have observed associations between adverse health outcomes in children and maternal exposure to OP insecticides during gestation, including abnormal reflexes (Engel et al. 2007), reduced cognitive abilities (Bouchard et al. 2011; Engel et al. 2011; Rauh et al. 2011), and ADHD and attention problems (Bouchard et al. 2010; Marks et al. 2010). Although OP insecticide use in the United States has declined in recent years (Grube et al. 2011), exposures to them remain widespread. These results are not definitive, but they serve to remind us about Rachel Carson’s warnings about the use of chemicals of uncertain toxicity, including OP insecticides. Ref Rencs Barr DB, Angerer J. 2006. 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