Babe, tell me to shut up if you want,i hope I'm not being insensitive but just putting it out there...is it possible that maybe you m/c the first pregnancy, and concieved again without knowing? It's a bit of a long shot but maybe that's why the dates are so weird (only possibility I can think of really, seeing as you were sure?)
I just hope that scan shows a healthy, squirming little bubba for you, SO much :hug)

