Duke hospital is a tertiary care center with 24 h inhouse anesthesia, obstetric, and pediatric staffs and a neonatal intensive care unit. In 1982, a fetal diagnostic unit was established and vaginal delivery after previous cesarean section was offered to parents. No administrative directives regarding cesarean section were made during the study, but all physicians knew the rate was increasing. Information was gathered from records of women who delivered after 20 weeks of gestation. Six major diagnostic categories for primary cesarean deliveries were defined. Risk factors preceding delivery by cesarean section were compared with those preceding vaginal delivery. Rate of primary cesarean delivery, odds ratio, prevalence of each risk, attributable risk (the difference in the rates of primary cesarean delivery between exposed and unexposed groups), population-attributable risk (the risk of primary cesarean delivery in the entire population minus the risk in the unexposed group), and the population-attributable risk fraction (the fraction of risk in the population that represents the excess risk associated with exposure to the risk factor) were derived statistically. Logistic regression was used to assess changes in the odds ratios of risk factors by the year for those factors with odds ratios > 1.5, or a prevalence and a population-attributable fraction > 5%.