Hi. I had a baby last year and I'm De Facto on my partner 457 Visa.
I only paid for the very 1st scan then all the antenatal care was done at the hospital I had been referred to.
I had to have lots of extra scans and monitoring for baby and there was no problem. You need to come from the UK though to have reciprocal agreement and benefit from Medicare.
You need to consider the non medical cost as well : nurseries (if you are going to work - no rebate if you are on 457), equipment (prams etc can be expensive), clothes. However you can get good deals if you go to 'Baby Markets' where they sell clothes, prams etc second hand , usually in really good condition and only a few dollars!
Good Luck.