I am not sure, but I would, to be on the safe side, drink the water and then if you didn't need to, at least you could empty the bladder before the exam. Better that way than get there and be told they can't do the u/s because your bladder is not full! :)
Maybe each Dr. is different, or it's different for the reason you are getting it, but with my last pregnancy I had to have a total of 12 level 2 and before each scan the lady always asked me if I had to go to the bathroom or not. It didn't seem to matter if my bladder was full or not.
One tip, eat a snack before to wake your baby up so you can see him/her moving around. :)
I think it depends more on how far along you are. I'd come medium full - not topped off - but with a bottle of water just in case. Even for my 10 week u/s they made me pee (half way!) so things weren't smooshed. And that was an abdominal u/s. By 20 weeks, the tech told me the full bladder wasn't necessary.
I don't think the full bladder is necessary. They seem to be changing that. My instructions said to come with a full bladder and I was told when I got there to go empty it right away because the baby was squished! I would go with a semi-full bladder just incase. It doesn't have to be as full as you would think. Good luck!
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