I think you're going to have to wait til he/she pops out!
DD2 showed all signs of being a boy, everyone was convinced, we didn't find out the sex so all surprise but even with all the boy signs she was a girl!
So who knows as for the u/s it doesn't sound too convincing when he says its a girl cause he can't see a penis, maybe it was facing wrong way or something? Was there anything to indicate girl besides no penis?