I had mine on Tues at 11am and they told me to empty my bladder at 9.30am, then drink a litre of water before 10am and hold it til the scan. This is also what they told me to do for the 12 week one. Pure torture I reckon! My Ob had told me to make sure my bladder was really full for the 20 wk scan too so they could get a good look at the placenta?

Wish I knew if I didn't have to drink the whole litre because I was pretty uncomfortable but the sonographer let me go to the loo about half way through as she said the rest of the measurements could be done without a full bladder?

Sorry I couldn't be of more help but good luck and enjoy