When my sister had her little guy, the ultrasound told her that he would most likely be on the smaller side (I think it said he was 5lbs at the time, and this was only a few weeks before she was due), and he ended up being 9lbs 4oz! She was worried for nothing. So, I really don't tend to trust what the ultrasound says.
As far as the baby getting stuck, I don't think your ob would let that happen, they should be able to tell before you begin pushing the approx. size, and if they feel you won't be able to get him out, they will do a c-section. Good luck!