I've got nothing to add to Dana's point here, but it really is impossible to imagine the outcry that would occur if Barack Obama had a pregnant, 17-year-old daughter. My sense is that Bristol Palin's pregnancy is being understood as a family matter, and Sarah Palin's compassion and commitment to her daughter is shining clearly through. That's as it should be. But if there were a teen pregnancy in Obama's family, it would end up confirming something altogether different, and much more sinister, even if the circumstances were exactly the same.
