I forgot to ask my midwife so looked on the internet and think I found something that recommended 2 pints before. If you're struggling to hold all that in, drink some of it whilst you're in the waiting room.
I don't know how much it is essential though - they told me to go and empty mine half way through the scan to dislodge the baby as it wasn't lying in a decent position (flat from head down spine) for them to check the spine. Apparently even this early, when you pee it rouses them and wakes them up!
Good luck - hope you get some good pictures!