It's difficult to judge. Depends what time of day it is and whether you've had much to drink already in the day. It's a bit late for me to give this advice, but because my first scan was 2.30pm, I gave my bladder 'trial runs' in the days before the scan, having a certain amount to drink at lunch, emptying my bladder at 1.30pm and seeing how much I needed to sip before I was just right at 2.30pm. Sounds a bit crazy and obsessive, but it worked for me! I think I had about 400ml two hours before scan, then I emptied bladder an hour later, then drank another 400ml or so in between last wee and the scan. I'd bring a bottle of water with you and don't drink too much before the scan, but if you're not feeling full enough fifteen minutes before, take a drink. Good luck x