a huge believer in skull theory over here!
With my son there was no nub visible, so guesses were based on skull only - every single person on two websites guesses boy, and 8 weeks later they were proved right. Now I know this isn't true for everyone but all the same, it's fun to guess...
For me, I don't like to guess after about 15 weeks with skull, to me the skulls begin to look the same around then, but I'm throwing a boy guess out there anyway!