OBJECTIVETo identify predictors of cocaine exposure during pregnancy, using hair and urine assays and self-report, and the association with preterm birth.METHODSA nested case-control study was conducted in a cohort of 2611 black and white women enrolled in prenatal care in central North Carolina. Cocaine exposure was ascertained by self-report (263 cases, 612 controls), urine assays at 24–29 weeks' gestation (226 cases, 564 controls) and postpartum (160 cases, 408 controls), and postpartum hair assays (169 cases, 435 controls). The major metabolite of cocaine, benzoylecgonine, was measured in urine. Cocaine and benzoylecgonine were measured in hair.RESULTSCocaine exposure was identified in 2% based on self-report, 5–6% based on urine assays, and 13–15% based on hair assays. Black ethnicity, lower education, and poverty were strongly predictive of positive hair assays. Hair cocaine and benzoylecgonine were not associated with preterm birth, with the possible exception of higher levels of cocaine and benzoylecgonine and birth before 34 weeks' completed gestation. The urine screen at 24–29 weeks' gestation also gave some indication of a possible association (odds ratio 1.7, 95% confidence interval 0.9, 3.5).CONCLUSIONThe study corroborates the incompleteness of cocaine exposure assessment by self-report and urine screens relative to hair assays. The strong demographic predictors of exposure suggest where intervention efforts should be targeted. The most sensitive markers of exposure, hair cocaine and benzoylecgonine, are not associated with preterm birth, perhaps because they reflect different patterns of cocaine exposure than the other measures.