Am I the ONLY one who thinks this?
I skimmed the topics here and I realized that most of the people who watched this movie think that the baby is some sort of vampire/zombie that came back from the dead. However, that is not the interpretation that I have made from this movie.
Here's my less literal version of what happened in the movie:
The baby was dead all along. The mother brought home a dead baby. Remember how the flies were attracted to the baby? Yeah, that's because flies love decomposing flesh. Yummy.
When the mother came in to change her diaper, she commented on a bad smell, but there was no poop in the diaper. It was clean. The mother also commented on how Grace is such a good baby because she never cries. This supports the fact that Grace was dead all along.
No one even saw the baby, really. The mother isolated herself from the rest of the world. My theory is, that when Grace didn't drink mink (Dead babies can't drink or eat, you know), she started to push on her breast in a desperate attempt to get Grace to drink, and when it drew blood, she formed the conclusion that Grace needed blood to survive.
But in reality, the baby wouldn't need blood or milk, or anything. She was simply going insane and trying to justify everything by finding a very odd solution, which in her mind, probably made sense. She killed two people, after all. The woman was crazy. She brought home a dead baby, fed it blood, and killed people. She was simply going nuts over losing her child, so she made up this fantasy of the baby being alive, and staying alive on blood.
The woman who ran off with her loves her and is playing along with her fantasy because she wants a relationship. When she commented on the baby teething, I don't think that she was really teething. She probably mutilated her own breasts in an attempt to make her fantasy seem more real.
So, I think that this was all in her head. Don't criticize me and tell me that I'm stupid and completely missed the mark, please. :)