I only had one once for an IUD I had inserted. I have never been pregnant. Was asked to drink 1L of water before the appointment, and they ran 45 minutes behind. I was livid.
The kicker was when I got there, the tech said "You didn't need to drink all that water- you're slim enough we could have gotten a good enough image." Or, something to that effect. This was 5 years ago. The part I did remember was when the tech said "You can go, you are way overfull." and RAN to the washroom to take a 90 second gushing pee. Best/worst hold of my life considering it was in public and I just wanted to