"The Back-Up Plan" hopes to generate romantic comedy by reversing the normal order of things so that pregnancy comes before a couple meets. Otherwise, it's still boy-meets-girl, and the only issue between them -- because it's more or less a love match right away -- is how the boy feels about the girl already being pregnant. Because "The Back-Up Plan" is closer to an old Doris Day comedy than "Sex and the City," that pregnancy is the result of artificial insemination rather than a complicated relationship with anyone else.