What is the nub/skull gender theory?

[SweetSunflowerBoo]

I have my 12 week scan tomorrow

Just wondering what the nub
And the skull
Theory is for guessing gender?

Thanks

Bump

Google it, there's loads of info out there!

The theory is that a nub that rises/sticks up is indicative of a boy and a nub that is straight/parallel with the spine is a girl

Skills are bit harder to define as info can be a bit conflicting

Thanks! @andivfmakes3