I got a leaflet from my midwife when she booked the appointment and it said to drink 2 pints of water an hour before. I did this but needed to pee after about half a pint but managed to hold it in and drink another 2 pints my bladder was very full though, I had to go and empty it a bit half way through the scan lol.
I am sure everything will be fine, Id say about a pint to 2 should be enough. She said I wouldnt need to have a full bladder at my 20 week scan. It will be amazing seeing your little been moving around on the screen, even if you are dying to wee!!
I had my 12 week scan today and it was amazing...enjoy! x