As far as I can tell from spoilers I don't think this baby scandal is Blair's but I don't think it can be Serena's either who could be the father? More to the point I have a feeling it will be Lily's, if there aren't already enough problems already in the Serena - Dan relationship I feel the producers just need more problems and one little baby sibling they share and see all the time would surely put a dampner on things.
What do you think?