Nub theory what is it?

[songbirdnumber1]

Hi all had our dating scan today and have heard about the nub theory but unsure how or what is it? Here's a photo of peanut

Anyone?......

I think that's a boy...Not sure how to explain nub theory but I think it's the straighter the nub, the more likely it's a girl.

I keep looking and can't even see a nub so have no idea how your meant to do it think we'll just have to wait for our '
Surprise'!