Recent increases in cesarean birth rates are of concern to obstetricians and to the public. Previous studies often focused on indications for abdominal delivery, described in such broad categories as repeat cesarean, breech presentation, dystocia, and fetal distress, without detailed information concerning medical/obstetric risk factors. In this study of 2744 consecutively delivered mothers who received antenatal care, the problem was approached differently by evaluating approximately 100 pregnancy risk factors recorded in a computerized uniform perinatal record system before making a decision for delivery. Excluding repeat cesareans, relative risks for primary cesarean birth were calculated for each risk factor. For the large majority of patients, the need for primary cesarean birth could not be predicted before the onset of labor. Approximately 70% of primary cesareans could be accounted for on the basis of single pregnancy risks—antepartum problems, 11%; abnormal fetal presentation, 30%; abnormal labor, 14%; and fetal distress, 15%. This leaves 30% of primary cesareans without a single major preceding risk. Along with the facts that 80% of primary cesareans in this series were performed for normal-size infants and that dystocia appears to account for a 3.4-fold greater proportion of cesareans in the national experience than in the current study, this suggests that critical evaluation and study of obstetric management of dystocia may be most fruitful in responding to the question of rising cesarean birth rates.