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Obstetric brachial plexus palsy (OBPP), is an injury of the brachial plexus at childbirth affecting the nerve roots of C5–6 (Erb-Duchenne palsy—nearly 80% of cases) or less frequently the C8-T1 nerve roots (Klumpke palsy). OBPP often has medicolegal implications. In the United Kingdom and the Republic of Ireland the incidence is 0.42, in the United States 1.5, and in other western countries 1 to 3 per 1000 live births.Most infants with OBPP have no known risk factors. Shoulder dystocia increases the risk for OBPP 100-fold. The reported incidence of OBPP after shoulder dystocia varies widely from 4% to 40%. Other risk factors include birth weight >4 kg, maternal diabetes mellitus, obesity or excessive weight gain, prolonged pregnancy, prolonged second stage of labor, persistent fetal malposition, operative delivery, and breech extraction of a small baby. OBPP after caesarean section accounts for 1% to 4% of cases.Historically, OBPPs have been considered to result from excessive lateral traction and forceful deviation of the fetal head from the axial plane of the fetal body, usually in association with shoulder dystocia, which increases the necessary applied peak force and time to deliver the fetal shoulders. Direct compression of the fetal shoulder on the symphysis pubis may also cause injury.However a significant proportion of OBPPs occurs in utero, as according to some studies more than half of the cases are not associated with shoulder dystocia. Possible mechanisms of intrauterine injury include the endogenous propulsive forces of labor, intrauterine maladaptation, or failure of the shoulders to rotate, and impaction of the posterior shoulder behind the sacral promontory. Uterine anomalies, such as fibroids, an intrauterine septum, or a bicornuate uterus may also result in OBPP.It is not possible to reliably predict which fetuses will experience OBPP. Future research should be directed in prospective evaluation of the mechanisms of injury, to enable obstetricians, midwives, and other health care professionals to identify modifiable risk factors, develop preventive strategies, and improve perinatal outcomes.Obstetricians & Gynecologists, Family PhysiciansAfter completion of this article, the reader will be able to summarize known risk factors for shoulder dystocia, describe the relationship between shoulder dystocia and obstetrics brachial plexus injuries, and describe three potentail explanantions for brachial plexus injuries other than lateral traction at delivery.