I've just had my 12wk scan and was told a theory on gender prediction by the sonographer that I've never heard before.
Apparently research is underway at the moment to investigate if male/female brains develop differently in the foetus. So far (he reckoned) evidence suggests that in boys the left side develops more rapidly than the right and vice versa in girls. As you can see the right and left sides very clearly by 12 weeks and they are measurable, if the theory proves to hold water then there could be a way of knowing the gender quite early on. He then quantified this with "but of course it could all prove to be rubbish too!"