Help with determining baby sex through nub theory

Hi all,

I've just had a private 13 weeks scan and the sonographer told me that they knew the sex of my baby but was unable to tell me ! They said that I would have to pay for a 16 week and come back!

I was hoping someone would be able to help me with telling the sex by using their knowledge of the nub theory ? I've tried but can't make heads or tails of it !

Thanks for your responses! X