The relationship of fetal position and ethnicity with shoulder dystocia and birth injury.

The objective of this study was to examine factors associated with the occurrence of shoulder dystocia and subsequent perinatal outcomes. We conducted a retrospective cohort study of 29,612 consecutive term, singleton, vertex vaginal deliveries. The primary outcome was reported shoulder dystocia. Fetal position, ethnicity, and their interaction terms were examined along with maternal characteristics, induction and length of labor, operative vaginal delivery, epidural, and birth weight in both bivariate and multivariate analyses. Among women who met study criteria, 524 (1.8%) experienced a shoulder dystocia. African American women had the highest risk of shoulder dystocia (2.6%), compared with other races/ethnicities (P = .001). Women who delivered in occiput posterior position were noted to have a lower risk for shoulder dystocia (1.1%) as compared with occiput anterior position (1.8%, P = .046). However, in the setting of a shoulder dystocia, a higher risk of brachial plexus injury was observed in neonates delivered in occiput posterior position (adjusted odds ratio 10.4, 95% confidence interval 3.03 to 35.88) by vacuum-assisted vaginal delivery (adjusted odds ratio 3.24, 95% confidence interval 1.37 to 7.67) and neonates weighing 4000 g or more (adjusted odds ratio 2.53, 95% confidence interval 1.09 to 5.85). Overall African American women have an increased risk of shoulder dystocia, but their neonates are not more likely to experience birth injury. Although occiput posterior position has a protective effect for shoulder dystocia, the risk of brachial plexus injury is increased in the setting of a persistent occiput posterior delivery. These factors should be used to consider a patient's prospective risk for shoulder dystocia and associated outcomes.