Therefore, Gonik's original premise, that shoulder dystocia must have been present if Erb's palsy occurred, is not supported. This brings into question the cause of Erb's palsy in those cases without shoulder dystocia. The maternal forces are the most likely cause both with and without shoulder dystocia.

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A better understanding of the forces involved when brachial plexus injury occurs has evolved over the past 10 to 15 years. A particular challenge was that all of the useful information had to be derived indirectly by identifying associations-a challenge that was met by individual researchers who made significant observations that, in turn, stimulated others to search for additional findings. Gradually the pieces of the puzzle began to form the picture. The significant steps in this journey were first, the recognition of the substantial number of injuries occurring without concurrent shoulder dystocia; second, the finding that a much greater frequency of injury is associated with an ultrashort second stage of labor; third, the observations that the injury rate is independent of the experience of the birth attendant; fourth, the recognition of the substantial numbers of injuries occurring in the posterior arm; and fifth, the anecdotal experience of countless delivery attendants, who relate that the forces applied in the injured cases were perceived to be no stronger than those applied when no injury occurred. We present a historical review for each step in this journey. We reviewed all articles published on this subject in Obstetrics and Gynecology and the American Journal of Obstetrics and Gynecology and some European journals. The period for review primarily covered articles published since 1980. Several textbooks on obstetrics and child neurology were also reviewed.

In the past decades, obstetric personnel have attempted to reduce the frequency of brachial plexus injury (BPI) by attempting to use less traction during the delivery of the fetal anterior shoulder. Clearly, these attempts have been unsuccessful in preventing or reducing the frequency of BPI. We conducted a nonsystematic literature review of the reported controversies regarding the mechanisms of BPI causation and why this complication is not decreasing in frequency. The review indicates that maternal labor forces are the most likely cause of BPI.

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