20 week scan

My doctors appointment was last week I’m 18 weeks now and my OB hasn’t scheduled me for a sonogram yet for my 20 week scan she just made an regular appointment for my next visit which is in 4 weeks I’ll be past 20 weeks by then . The first time I scheduled an ultrasound it had to be at the main hospital and not the clinic that I go to for regular visits. I’m really anxious to find out the sex should I call her to schedule a sonogram? Or just wait until my next appointment