We go once a month for just a normal follow-up and to listen to the heartbeat. Our 20 week A/S is at the hospital and I won't be seing the doctor either. We meet with the doctor the following weekend to discuss everything - that's just the way our once a month appointment happened to fall.
Your appointments sound normal because you are going about every 4 weeks. It's okay that you aren't seeing your OB doctor at the 20 week A/S because your doctor will still get the results of everything.