Tell the sonographer as soon as you walk in the door that you don't want to know! We didn't find out with DS and I had to have a few extra scans due to placenta praevia so every time I walked in, I said "I don't want to know the sex!" Never had a problem with any of the sonographers letting it slip and I could never even work it out looking at the screen. We even got a DVD of the 20 week scan and I still couldn't tell.