I went through shoulder dystosia with DS2. I believe it was a combination of size, birthing position and the speed of my labor 2hr45mins from first contraction to birth. As a result of the technique they used to get him out I ended up needing back surgery and he was born not breathing and his poor head was swollen and bruised from the ob pulling him put so when it came to DS3 and he was predicted to be bigger I opted for a c/s. DS2 was 9lb 1oz and DS3 was 9lb 4oz.
In hindsight if I knew about different birthing positions etc it may not have happened.