OK, they say that Sarah Palin doesn't look pregnant. But, if their whole timeline is correct, Bristol would be 7 months pregnant at the time of that picture, andshe doesn't look 7 months pregnant (honestly, shoe looks normal, just a little bit of a belly, like a LOT of teenage girls.
Neither of them looks pregnant to me.
I'm old enough to have seen many pregnant young women in sweaters, and I have some years perspective on the HS crowd. However I now have a daughter in HS, and so I also have the current opportunity to see many of her friends. FWIW Bristol looks pretty normal there and not pregnant.