Shoulder dystocia results from a persistent anterior-posterior position of the fetal shoulders during fetal descent in labor. Normally, the fetal bisacromial diameter (the distance between the outermost points of the fetal shoulders) enters the pelvis at an oblique angle. The shoulders rotate to an anterior-posterior position with external rotation of the fetal head, which allows the anterior shoulder to slide under the symphysis pubis. With shoulder dystocia, the shoulders remain in an anterior-posterior position during descent, or descend simultaneously (rather than sequentially), leading to impaction. Either anterior impaction under the symphysis pubis or posterior impaction behind the sacral promontory can occur. Increased resistance between the fetal skin and vaginal walls (as in cases of macrosomia), or cases where truncal rotation does not occur (as in precipitous labor), can lead to shoulder dystocia. In some cases, stretching of the nerves in the brachial plexus during labor and/or fetal descent can result in nerve injury.

Numerous risk factors have been described for the occurrence of shoulder dystocia. Even though about 50% of cases occur in infants weighing less than 4000 gm, the incidence of shoulder dystocia increases progressively as birth weight rises. An infant weighing between 4000 and 4500 gm has an 8–10% chance of shoulder dystocia, as compared to a 20–30% chance if the infant weighs more than 4500 gm. The most commonly used threshold for macrosomia is a birthweight over 4000 gm. Of note, fetal body configuration may be more important than birthweight per se, as macrosomic infants have a trunk or chest circumference larger than the head circumference in addition to an increased bisacromial diameter. These factors impede normal shoulder rotation and can lead to the shoulder impaction.

Diabetes mellitus is another risk factor reported in association with shoulder dystocia. Many macrosomic infants are delivered from women with diabetes. As such, the incidence of shoulder dystocia in diabetic mothers is higher than that in the general population (9–33%). Moreover, infants of diabetic mothers experience a significantly higher shoulder dystocia rate than infants of nondiabetics of a similar birthweight. ACOG advises against a trial of labor in patients with maternal diabetes and an estimated fetal weight > 4500 gm.

Other historically reported antepartum risk factors for shoulder dystocia include postdates gestation, advanced maternal age, multiparity, excessive maternal weight or weight gain, oxytocin use, epidural use, and prior shoulder dystocia. Intrapartum risk factors include a protracted first stage, prolonged deceleration phase (between 8 and 10 cm), and epidural anesthesia.

Attempts at predicting the occurrence of shoulder dystocia have been disappointing. Even though it is evident that shoulder dystocia rates increase with increasing birthweight, efforts to predict birthweight accurately with antepartum or intrapartum assessments remain very poor. Ultrasound estimates of fetal weight are often no better than clinical estimates in predicting macrosomia, and have an inherent error rate of +/− 15% in estimating the actual birthweight. Furthermore, weight estimates become increasingly inaccurate as birthweight increases. As such, numerous studies have demonstrated that while many of the risk factors noted above are indeed associated with a higher incidence of shoulder dystocia (macrosomia in particular), their positive predictive value remains very low (1–3%). Studies have demonstrated that induction of labor for suspected macrosomia (as compared to expectant management) does not decrease the incidence of shoulder dystocia or brachial plexus injury and only results in an increased rate of cesarean delivery. Thus, in practical terms, the presence of one or more risk factors should lead to a heightened awareness of the chance for shoulder dystocia, but elective cesarean section for suspected fetal macrosomia should be reserved for cases with an estimated fetal weight > 5000 gm (4500 gm in diabetics).