Hi Julie, that entire scene is very much open to interpretation. JKR is dealing with the spiritual there and being far from precise. My interpretation is that the baby was Voldemort's soul, or the little that was left of it after 7 parts were destroyed. There is support for that in the book because in the last battle with Voldemort Harry shouts something to the effect that he's seen Voldemort's soul and that it's a wreck. However, like I say that whole chapter is very much open to interpretation.

Bart.