im sorry your OB has put this doubt in to your mind!
You are right that your body grows a baby right for your body!

check out the gaskin manouvere(sp) maybe give the info to your ob
it is a change of positon that opens your pelvis up more when bubs is (if it happens) stuck.

I think you know that no good will come out of the scan as they can be soo far off base with the measurements. that i think you shouldnt go....
as it will add more stress and doubt on to yourself about "not being able to birth" your baby the way you want! and that your ob obviously thinks you wouldnt be capable either.