They can't always tell you the sex at the 20 week scan though as sometimes bub is positioned in such a manner that they can't see between the legs. Regardless of when you get it done ultrasound is not 100% accurate so it could be wrong. My neighbour had 8 scans during her pregnancy - all said boy and it turned out to be a girl. The only way to know for absolute sure is via CVS or amnio and you'd only get those done if you have a bad NT scan result at 12 weeks (if you are having that done).