I had to do that for my 19 week scan, but I'm not sure why it is. I had to down 3 glasses of water (750ml) an hour before the scan. It was very uncomfortable, to say the least - especially when you have a little baby sitting on your bladder already! But once I got into the scan, I was so mesmerised by seeing my little one up on the screen that I managed to forget all about my full bladder.

I don't know why you have to have a full bladder for the scan - but I was advised specifically to do it. I figure there must be a good reason.

Have a fantastic scan!!!