My sister was told girl by one sonographer at 17 weeks and started buying an obscene amount of girl things... 20 weeks rolled around and a different sonographer swore blue it was a boy and showed her all the correct boy parts, but explained that the cord could have been in the way etc etc. Apparently when they say boy they're so much more confident because we all, inherently, start out by looking female and each baby grows at a different rate.