Obgyn requested delivery payment upfront

I'm looking for advice, I am 32 weeks, and just received a call from my obgyn Saying that I am due to pay $600 ( my 20%, insurance covers 80%) before the delivery. I am just wondering if that's a normal practice?  They didn't send me an invoice, and said I won't recover one as they got the price after discussing my cover with Aetna... It's all fine but we just wanted in wr tiring, so we know exactly what we are paying for ( just the delivery/after care)