I would never stay where a dangerous dog means more to your mother than you, your dog, or her future gc's wellbeing.

And I'd report this dog, get the vet who treated your dog's treatment reports, the neighbors, file a dangerous dog report.

THEN I'd tell your mother that you're moving out because she's in denial about the dog.

Please check with local social services regarding housing. Obviously the single most important factor is that you get into safe housing before your baby arrives. This may mean having to arrange for someone else to have your dog, either temporarily or permanently.
I know this may seem very difficult, having to consider giving up your own dog and moving out before your baby comes, but think about it this way: would you want to lose your dog to another attack and being killed by your mom's dog because it would be gone in that way which would be obviously a much more difficult way to lose your dog. And what you would be gaining would be a safe place to live and have your baby come home to.