I thought I'd update this thread in case someone is looking for info on this.
I'm with Medibank and they assured me that if my EDD was outside the 12 mths waiting period I'd be covered regardless of when baby is born
Turned out that my EDD is within the 12 months waiting period by 2 weeks.
At first they wouldn't bulged, saying the waiting periods were very strict etc. But after a few email, phone calls and lots of tears they finally agreed to back date the start of my obstetric cover.
So now my EDD is outside the waiting period.
Very happy with Medibank!!