There is 3 group of injury of plexus brachialis as a birth trauma.
Only 30% of all injured recovers from it totally and with small help from physical therapy in term of few days to few weeks (4-6) and lately to 3 moths of child life. That is smaller injury called neuropraxis (reason for interruption of neuron conduction is swelling and hemorrhagic).
The major number of injured children, about 55%, recovers in period of 6-12 months and lately to 18 month of life. That is axonothmesis. Finally recovery of this lesion may be different. Few of them recover with minimal weakness of muscles and discrete decrease of coordination. Group of children who show first sign of active movement after 3 or more months very often after first good results start to show characteristic manifestation: muscular-ligg. contractures mostly on shoulder region and elbow, but lately in life on distance segments (spine). Affected arm usually is shorter in length and velocity and weaker muscles strength, with appearance of contractures, co-contractions of antagonistic muscles (biceps-triceps) and paradoxes synergies (deltoideus-teres major), and no using arm in postural reactions and in daily activities. All this is very notable after 2 year of child life.
Smallest, 15%, of the injured children are the children with low or no recovery at all. That is either neurothmesis (total break) or avulsion of routs. No need to tell how much that affects motor and sensor conduction and how big that is handicap for the child.
So, positioning in first two weeks after birth, some mild thermo procedure like introduction in kinesytherapy- what is by my modest opinion the most important for the recovery (with using passive motion for full ROM and stimulation of active motion), yes you could use electro stimulation after 3 week of life one time per day (you could use before longitudinal galvanization as an intro procedure for stimulation).
I hope this would help. If you have more questions about that feel free to ask, I have 5 year experience in childe rehab and be glad to help if I could.
Best regards Nale