Shoulder dystocia is an infrequently encountered obstetric emergency varying in incidence from 0.15 to 0.60% of all deliveries. Previously identified risk factors include maternal obesity, previous infants weighing greater than 4 kg, maternal diabetes, and fetal macrosomia (> 4 kg). To evaluate the role of prolonged second stage of labor (PSS) as a warning sign for shoulder dystocia, 9864 deliveries at LAC-USC Women's Hospital were retrospectively reviewed. Ninety percent delivered vaginally and 4.89% had PSS with midpelvic delivery. Shoulder dystocia occurred in 0.37% of all vertex vaginal deliveries. In the absence of PSS and midpelvic delivery, the incidence of shoulder dystocia was 0.16%. However, with PSS and midpelvic delivery, the incidence of shoulder dystocia was 4.57% (P < 0.01). Infants weighing in excess of 4 kg were at increased risk of shoulder dystocia compared with infants weighing less than 4 kg. When PSS occurred and midpelvic delivery was attempted, the incidence of shoulder dystocia was 21% in infants weighing in excess of 4 kg; 8% had had failed vaginal delivery. All shoulder dystocias and failed vaginal deliveries occurred after use of the vacuum extractor. Immediate neonatal injury was apparent in 47% of infants with shoulder dystocia after PSS with midpelvic delivery. There were no maternal or fetal deaths related to shoulder dystocia during the study period.