What is the Nub and Skull gender prediction theory?

[Armywife726]

Hello all,

I am new to the site, and while skimming through the threads I noticed many people asking for gender predictions based on Nub and Skull in profile pictures. I have never heard of this theory before and I was hoping someone wouldn't mind explaining what you look for in the picture and what points to the boy or girl guess? I am 16 weeks, but had an ultrasound at 12 weeks and crossing my fingers for a girl. Any feedback is much appreciated, thank you in advance