Warren, to his credit, didn't pose a metaphysical question, or a biological one. He asked a legal question: "At what point does a baby get human rights, in your view?" Obama tried to dodge by saying that from a "theological perspective" or a "scientific perspective" the issue is "above his pay grade." But Warren asked a more narrow question, and one that any politician who votes on abortion laws should be able to answer. And of course, as a supporter of Roe and Casey, Obama does have an answer: He thinks that a baby acquires rights when it's born - well, perhaps depending on how and why it happens to be born - and lacks them at every juncture before birth. He just didn't want to come out and say it.
Legally, this issue is moot as long as Roe is in force. Theologically, it is a thicket, as Ross understands. I think Obama's position was completely defensible and more thoughtful than McCain's. But less effective.