I think most offices have you schedule the u/s ahead of time. You could definatley have one at 18 wks though. I didnt have an 18wk check up but at my 16wk it was just the normal stuff. It will be the same at my 20wk check up
It all depends on the hospital you go to. With my DS and DD, I had ultrasounds at my 16-18 week appointment. However, I just went to my appointment for this pregnancy about a week ago, and I had to go to a different department to schedule my ultrasound.