I knew on my first visit I had to pay $1500 to ob or else they would drop me. I made payments every time I went in. I would talk to them but honestly it really sounds like they have told you about it often enough and you should have been trying to.work something out and/or paying something.
You might want to find out if you are going to have a copay at hospital too. We owed $6000 after dd4 was born.