I'm kind of surprised that some suggest the "I feel so bad for her" line. I would go with complete silence and just raise my eyebrows a bit. It'll make supervisor think twice about what she just said/implied. If she escalates at all, and says something like, "I don't know why you need 12 whole weeks", then I'd go the medical information route ("I'm sorry, but that's really between me and my doctor.") and then notify HR.