My son broke his collar bone at 18 months when he tried to lean and reach the dog. The dog moved and my son fell. We were told not to lift him under that arm (not that you'd lift a newborn that way) and to pin a colorful square of fabric to his clothes near the shoulder on the affected side to remind everyone to be careful with that shoulder. It seemed to heal very quickly, in a matter of 2 weeks.
I don't know how common this injury is in newborns.
Hope this helps a little. Barbara