My OB showed us girl parts with DD, but still said his disclaimer that U/S isn't 100%. With DS, it was obvious the way he was positioned. We tried peeking at this one several weeks ago (as part of U/S, not specifically to determine gender), but OB said nothing was developed enough to judge if parts would turn out to be one or the other. My Level II is a few weeks away, I hope baby grows a lot and lets us get a good peek!
I would tend to believe the later U/S for that reason- that baby is bigger and so are key parts. Either way, congratulations!