I just had my first scan and had to have a full bladder I drank 1/2 a litre of water but had to go to the loo after doing it - I'm going a lot at the moment - and then drank the other half and figured I'd be OK. When I got there she tried to do the test but couldn't do it as my bladder wasn't full enough. She sent me out to reception where I had to wait half an hour by which time I was busting. She then managed to do it. I've never heard of anyone being told not to drink water before an ultrasound so if I were you I'd ring and check that the info was correct. You don't have to have a full bladder with the internal ultrasound.