I've read that you should send a t-shirt home from the hospital ahead of the baby (one that the baby has worn and carries his/her scent). I've also heard people say that placing this in the dog's bed or special place will give the dog the idea that the baby is considered alpha.
I have NO idea if this is true or not. It makes sense to bring the shirt home. Can't hurt, might help.
My personal feeling is that it has a lot to do with the temperament of the animal. Some will ignore the baby and some will feel protective from the get go.