your invitation to apply for visa 189 will expire after 60 days
you wouldn't know when will you be invited again (if you decided to let it expire)
there might be changes in the future (fee/sol/csol/other requirements)
so i suggest, you apply immediately, and go to the accredited doctors..
if they will allow for xray (early days of her pregnancy) it might be possible..
otherwise, if it is not possible, then you really have to wait after giving birth,..
atleast you have already applied, and will not be affected by any changes in the migration rules...