For those of you that dont kno, the 'nub' is a way of predicting the gender from ur dating scan, if you google it youll get better info as its quite hard to explain, but I will try my best...
On alot of 12(ish) week scan pics u can see the urethra, and depending on what angle its at determines the sex of the baby... Pointing up a boy, down a girl.
If u look at my 2nd pic, there are 2 little lines around the bottom area, pointing downwards, this is the nub...
Obv this is only a theory, but it has proven to be quite successful