Risk factors for shoulder dystocia in the average-weight infant.

Almost half (47.6%) of all deliveries with shoulder dystocia occurred in association with the delivery of an average-weight infant (under 4000 g). Of 4294 nondiabetic gravidas delivering infants of birth weight 3500 to 3999 g, 94 (2.2%) experienced a shoulder dystocia. Protraction and arrest disorders were associated with a statistically significant increase in the incidence of shoulder dystocia, and this effect was further augmented by low forceps delivery. Among 6252 infants weighing 3000 to 3499 g, there were 40 instances of shoulder dystocia (0.6%). Only arrest disorders were associated with an increased rate.