The risk factors associated with the occurrence of shoulder dystocia were examined in the general obstetrical population of women delivering vaginally. An increasing incidence of shoulder dystocia was found as infant birth weight increased. Although one-third of shoulder dystocia occurred in pregnancies at 42 + weeks, except for those resulting in infants weighing 4500 + g, the vast majority was unaffected by shoulder dystocia. The incidence of shoulder dystocia in nondiabetic gravidas delivering an infant weighing 4000 to 4499 and 4500 + g vaginally was 10.0 and 22.6%, respectively. Within the 4000- to 4499-g group, no labor abnormality was clearly predictive; however, in the heaviest birth weight group, an arrest disorder heralded a shoulder dystocia in 55.0% of cases. Diabetics experienced more shoulder dystocia than nondiabetics. Among them, 31% of vaginally delivered neonates weighing 4000 + g experienced shoulder dystocia. Nevertheless, the risk factors of diabetes and large fetus (4000 + g) could predict 73% of shoulder dystocia among diabetics; large fetus along flagged 52% of shoulder dystocia in nondiabetics. Cesarean section is recommended as the delivery method for diabetic gravidas whose estimated fetal weight is 4000 + g. If others confirm the risk, the authors advise serious consideration of cesarean section for gravidas who are carrying fetuses estimated to be 4500 + g and who experience an abnormal labor.