I was told at DD1's 20-wk scan that the difference between boys and girls was determined by a certain number of 'stripes' (I won't spoil it for you by telling you how many!). So when we went to DD2's 20-wk scan, DH and I knew what to look for and we both saw the stripes that indicated a girl, and we were like, 'Bugger, we wanted a boy' lol. The u/s tech was astounded that we knew what we were on about, until we explained what we'd been told.

If you don't want to know, let the u/s know as soon as possible, and don't worry about accidentally seeing bits and bobs (or lack thereof) - it's all in the stripes!