Found this:
The nub theory is basically about 'the angle of the dangle'. Between 11 and 14 weeks' gestation both genders have a penis-like protuberance between the legs. They look incredibly similar at this point, except for the angle at which they are pointing. Sometimes there are males and females at this stage which are in the 'grey area', but essentially a boy's 'dangle' is 30 degrees up relative to the backbone and a girl's is below 30 degrees.
In order to be able to have any chance of predicting the gender, the foetus must not be curled up- they need to be lying as flat as possible. Also, it must be side on and apparently, it is better if the foetus is right side up.
So, check your 12 week scan picture!