How accurate is the "nub" theory?

[runnergrl]

oh wow, we have two new guesses among all the blue! Should be interesting. Funny it was at 100% boy for 18 posts!

 

[carmyz]

had my scan and its a girl..nub theory right for me

 

[Chasingkids]

Hi I'm new to this site but wanted to see if I could get opinions on my recent scan as to if it's a boy or girl based on the nub theory. This was done at 13 weeks 4 days. I believe the pic is attached.

 

[pinkpolkadot]

Hello runnergrl, I would say boy, nub theory was right for DD and DD2 on the way

 

[skye93]

Girl