I haven't gotten all the way through the thread yet, but Bristol became pregnant with Trig while her mother was governor and BEFORE her selection as McCain's running mate. Sarah gave birth in April, also before her selection as McCain's VP. When she was introduced in Ohio that August day, Bristol carried Trig in part so her own pregnancy would not be so noticeable.
Sarah was inviting Washington insiders to Alaska as early as summer 2007 to position herself for entry into national level politics, and was interviewed by the Washington Post in February about the possibility of being McCain's VP. If Bristol's baby was born in December, doing the math here, she knew her mother was in line for that nomination a month before she got pregnant.