Phenomenon of aberrant reinnervation ( misdirection of regenerated axons) is more frequently seen in the shoulder and elbow, but less in the forearm and hand in obstetric brachial plexus palsy ( OBPP). Dominant rupture of upper trunk with short gap and natural conduits by ruptured scalene muscle fibers are the main reasons causing aberrant reinnervation. One hundred babies°¦ palsies were surveyed based on intraoperative findings to try to find out the relationship between C5, C6 spinal nerves and quality of scar tissue, which contributes the sources of aberrant reinnervation. There were 8 types of lesion encountered: Type 1: C5 major + C6 major with upper trunk lesion and acceptable scars; Type 2a: C5 major + C6 minor with upper trunk lesion but worse scar on C6; Type 2b: C5 major + C6 minor with upper trunk lesion with C5 rupture and C6 avulsion/rupture; Type 2c: C5 major+ C6 minor with upper trunk lesion with C5 rupture and C6 true avulsion; Type 3a: C5 minor + C6 major with upper trunk lesion with worse scar on C5; Type 3b: C5 minor+C6 major with upper trunk lesion with C5 avulsion/rupture; Type 3c: C5minor+C6 major with upper trunk lesion with C5 true avulsion; Type 4: C5, C6 minor with C5, C6 avulsion. °µMajor°¨ here means dominant; °µminor°¨ less in sources contribution for reinnervation. The results showed type 1, 47 cases ( 47%); type 2a , 26 cases (26%); type 2b, 3 cases (3%); type 2c, 16 cases (16%); type 3a, one case (1%); type 3b, no case; type 3c, 2 cases ( 2 %); type 4, 5 cases ( 5 %). More than 90% ( 92%) OBPP patients their shoulder function was recovered by aberrant reinnervation. Half was come from C5 and C6 major contributions, and another half from C5 major and C6 minor contribution. Only 5% patients their shoulder function was paralytic type. Another new clinical finding was °µumbrella sign°¨, in additional to trumpet sign. °µUmbrella sign°¨ is a sign showing while doing shoulder elevation, the elbow will flex spontaneously, like holding umbrella. Severe umbrella sign, defined when shoulder abduction less than 90 degrees with elbow flexion, showed supposedly main sources of C5 major and C6 major, or C5 major and C6 minor grow to the anterior division of the upper trunk. This is so far the first paper presented to evaluate the phenomenon of aberrant reinnervation in obstetric brachial plexus palsy.