Oh we could guess on a skull at this point, it's just not reliable. I think it's better with the skulls, to compare to a pic of YOUR boys at this age. Comparing unrelated baby to unrelated baby is misleading because genetics vary so much, but comparing siblings to siblings may be helpful (my daughter def. had a different skull shape to her brothers)
It's too early for nub guessing though because a nub that looks pink at 12w2d can still rise and end up being a little man. I prefer to only guess nubs starting at the end of the 12th week at the very earliest and prefer 13th week. I have seen too many people end up heartbroken by a "girl nub" early int the 12th week that ended up being boys. So I just prefer not to guess at this stage. Good luck