Because, essentially, if Bristol's child is born is January, then the Palins' official timeline does not check out, making it physically possible that Bristol is in fact the birth mother of Trig, born in mid-April.
While I'm assuming that most babies are not born on their due dates--because it would be kind of crazy if we could literally predict the day everyone is born--let's not forget that it is December 28 already . . . .
HAPPY BIRTHDAY: Sienna Miller, who turns 27 today. May she have a lovely birthday and keep her shirt on in at least one film in 2009, namely G.I. Joe.