at 12 weeks generally even girls have nubs, and a guess is usually a 50%. so if they say boy and its right, those who had boys had just as much of a chance of it being wrong. although if they say girl it would be more likely to be right because there is obviously a lack of nub they are seeing. genitals usually start to distinguish in week 14. Although obviously babies do not always follow the same growth patterns. I had a scan at 15 weeks with my twins, they were very obviously boys. They couldn't tell me at 12 weeks. And anything before 12 weeks I would never, ever trust. I say 90% my butt. more like 50% sure lol