I feel your pain, trying to hold your bladder is the worst part, then to have someone push on your full bladder...It is VERY worth it though once you see your baby on the screen!
im having one at 8wks this wk:) but freaking out as im not sure i wll be able to hold the 1.5 ltrs that you have to in bladder ouch! for 1 hr :( im actually stressing about it :(
I had one at 5w and 6w because I was having cramping. My ob does one at 8w for dating and then not again until 16weeks.
Are you asking when should it be done?
They usually do an early ultrasound if they are unsure of your date, can be as soon as 6 weeks.