What is the skull theory?

[MissPlum]

I know the nub theory and I know the theory of the baby's belly looking swollen on the ultrasound pic it indicates a boy and if it is flat then it indicates a girl.
However I have never heard the skull theory and would like to know what it is as people are guessing My little beans gender using this theory

 

[saraaa]

This is the picture I go by boys skulls are more tennis ball rounded shape where as girls seem to have abit of a bump at the forehead xx

 

[mad_but_glad]

Here's a picture they use online. It is said to be 92% accurate going by skull! Left is boy, right is girl. Sorry that's my ultrasound pic underneath... I couldnt find my skull pic without it lol.

 

[mad_but_glad]

Ps going by skull I predict girl for you!