Concerns

So I had an outside ultrasound, my military hospital will only do one at this point if I move into a high risk pregnancy or there are complications, and they didn't know how many weeks I was when I got there, just that I was in my third trimester and they were measuring my boy and he's measuring at 31 weeks! Like, he bone growth is supposed to measure about the same amount of weeks you are or something, and now I'm really concerned. My doctor put my due date at December 15th but if my baby is measuring ahead should I expect an earlier birth? I'm a small person so my baby isn't likely to be huge at first so I can't imagine that he's just large. I'm not exactly sure what to make of it but I'd like to at least prepare in my head if I have nine weeks until birth as opposed to eleven. Opinions?