G3P2L2 at 40 weeks is in labour. The baby's head delivers but then retracts back against the perineum. McRobes maneuver is done to delivers the anterior shoulder.In postnatal period , the right arm is hanging & limp to the baby's side with the forearm extended and internally rotated. What is the likely reason ?

The above case history is suggestive of Shoulder dystocia for which McRobes maneuver is done Shoulder dystocias can be associated with significant fetal morbidity including brachial plexus palsies, clavicular fractures, and humeral fractures. Injury to the brachial plexus may be localized to the upper or lower roots. In Erb (or Erb-Duchenne) palsy, the upper roots of the brachial plexus are injured (C5-6), resulting in paralysis of the shoulder and arm muscles; the arm hangs limply to the side and is extended and internally rotated. In the case of Klumpke's paralysis, the lower nerves of the brachial plexus are affected (C7-T1) and the hand is paralyzed.