OB office wants me to pay for prenatal care/delivery up front

I'm 9 weeks, had my first prenatal visit last Tuesday. Today I received a letter for my OB office explaining my benefits and what my total cost will be after insurance. It says they collect this total from me up front and my full payment will be due at my next visit. This just seems strange to me, why wouldn't I pay for my care as I receive it? Is this how things are normally done?