Acquired brachial plexus injury historically has been linked with excessive lateral traction applied to the fetal head, usually in association with shoulder dystocia. Recent reports in the obstetric literature, however, have suggested that in utero forces may underlie a significant portion of these injuries. Brachial plexus palsies may therefore precede the delivery itself and may occur independent of the actions of the accoucheur. Thus we propose that the long-held notions of a traction-mediated pathophysiologic mechanism for all brachial plexus injuries warrant critical reappraisal. (Am J Obstet Gynecol 1999;180:1303-7.)