I was born in the UK in 1975. I was a breech birth, and my mother has always told me that the nerves in my arms were damaged when I was born. For the first few months I couldn't move my upper arms away from my sides - it was like they were stuck to me - and when she tried to do it for me it was incredibly painful. Eventually the doctor advised her to leave me to heal naturally, but surgery had been suggested at one point. Thankfully I seemed better after a few months. All I know was that during the birth my arms were reaching upwards and I was a forceps delivery so must have been stuck at some point.
I'm very lucky in that I did recover well and have good use of my arms. However my arms have always been considered by others to be quite weak. It doesn't cause me much of a problem, but for example it doesn't matter how many reps I do at the gym, I can never improve on lifting the lightest weights when I use shoulder or deltoid abduction/adduction machines. I also find it hard to move my arms behind my back like other people can. I can just about clasp my hands behind my back in some positions, but I don't seem to have the full stretch that most people do - it's quite uncomfortable if I try. I was also very late in developing fine motor skills, but I don't know if that's related
Like I said, I feel very fortunate that this doesn't affect my day-to-day life, but I was wondering if these symptoms along with the type of birth I have sounds like I have the symptoms of a Brachial Plexus injury? I'd ask my doctor but they're unlikely to send me for tests or diagnosis without very good reason (NHS) and it's really just curiosity about my birth and subsequent limitations that prompts me to ask.
Thanks in advance
I think 
you should seek this type of BPI doctor, just to hopefully resolve the weakness issue and the information you are seeking.