When I have ultrasounds for my babies (I don't know what yours is for) I drink somewhat under what they tell me to and I start later than they tell me to, because with my other two children I did exactly what I was told and they always said it was really, really full.  One time I had to wait an extra half hour or something because there was something going on unexpectedly and it hurt so bad that I had to let some out anyway.  So, I realized I was torturing myself for nothing because I was clearly drinking more than enough and after experience of being in there I know that it takes thirty minutes at the least before I am able to been seen by someone anyway.  I just make sure I drink while I'm there up until the last second, they always say I'm more than full enough.  

It sucks because they do keep you waiting while missing the fact that you are ready to burst but they don't really count the waiting time in the slot of time they give you.  smiley: laugh  They don't get that those extra twenty or thirty minutes can make you ready to scream.