Shoulder dystocia is an uncommon but serious complication of delivery. Clinically, shoulder dystocia may be diagnosed when delivery of the head is followed by an inability to deliver the shoulders. Shoulder dystocia generally requires additional maneuvers to free the shoulders and effect delivery of the infant. Although the exact mechanism is not well studied, the postulated mechanism is impaction of the anterior shoulder against the maternal symphysis pubis or impaction of the posterior shoulder on the sacrum. Rarely, dystocia may be the result of or may be made worse by impaction against the soft tissue of the birth canal.

The risk for shoulder dystocia is approximately 1 in 100 for normal-size infants. A variety of risk factors (see Table 23.1) have been associated with an increased risk for dystocia, including prior history of shoulder dystocia, known anatomic abnormalities of the birth canal, gestational diabetes, postdates pregnancy, macrosomia, and protracted labor. Although most cases cannot be identified on the basis of identifiable risk factors, infant size is clearly related to an increased risk for dystocia. Macrosomic infants (>4000 g) have a five- to tenfold increase in risk (absolute risk 5–9 %).

The complications of shoulder dystocia include direct trauma to the mother and/or infant, hemorrhage and, less commonly, possible complications of the delivery itself. Direct trauma to the mother may result in laceration, extension of episiotomy, and postpartum hemorrhage. Approximately 10 % of deliveries with shoulder dystocia result in postpartum hemorrhage (for management, see Chap. 28). Approximately 3–4 % of deliveries will result in fourth-degree lacerations or extensions of an existing episiotomy. Although the connection between birth trauma and subsequent neonatal outcomes is not clear, approximately 10 % of all deliveries complicated by shoulder dystocia will result in brachial plexus palsy. Of these, approximately 10 % will be persistent. An increased risk for clavicular fracture is also associated with shoulder dystocia.