I agree with the other comments, but I think, on a more literal level, this song could be about having a baby and really just not feeling like it's yours. I think the last verse is really great because of the twist Amanda puts on the fact that sometimes people don't know who the father of a child is, but almost always know who its mother is because she gave birth to it. But Amanda is saying that it doesn't really matter who the father of it is, just that it was something inside her that someone put there (I agree with the above poster) and she knows that its not hers because she feels like it doesn't belong to her.
I agree with the other comments, but I think, on a more literal level, this song could be about having a baby and really just not feeling like it's yours. I think the last verse is really great because of the twist Amanda puts on the fact that sometimes people don't know who the father of a child is, but almost always know who its mother is because she gave birth to it. But Amanda is saying that it doesn't really matter who the father of it is, just that it was something inside her that someone put there (I agree with the above poster) and she knows that its not hers because she feels like it doesn't belong to her.