Lopez not at her best in clichéd ‘Back-Up Plan’

“The Back-Up Plan” is about as much fun as 36 hours of labor, only you don’t get to go home with a baby at the end.

Instead, you leave with a throbbing headache and a lot of questions about why anybody still thinks Jennifer Lopez can anchor a movie. After all, “Out of Sight” was a long time ago.

Lopez plays Zoe, a gorgeous New York businesswoman who is inexplicably single (as all gorgeous women tend to be in romantic comedies, at least for the first hour and a half). Zoe desperately wants a baby, and as the movie opens she’s being impregnated with sperm from an anonymous donor.

On her way out of the doctor’s office – after she does the requisite little twirl on the street to indicate joy – she hails and gets into a cab. A hot guy (Alex O’Loughlin of TV’s defunct “Moonlight”) jumps in at the same time.

Naturally, they squabble, meet again and eventually fall in love, forcing Zoe to confess she’s already pregnant – an announcement that will lead to terror, misunderstandings and a tiresome amount of bickering about trust and commitment.

If there’s a cliche available for any given moment, screenwriter Kate Angelo latches onto it. Even the gag-inducing opening credits, in which a cartoon Zoe sees babies everywhere she looks, feel worn beyond belief.

Zoe is a former career climber who left the corporate world to buy a pet store from the mean puppy mill from which she bought her dog (no word on where she gets the dogs she sells).

Stan, the hot guy, is an organic farmer who makes fancy cheese and has green dreams of opening a sustainable food shop in Manhattan. He also has a nice farm upstate.

The list goes on. Zoe has a sassy grandma (Linda Lavin). A dog eats a home-pregnancy test. A wacky group of single moms hosts a birth in a living room, complete with chanting and drums.

You can predict with alarming accuracy when Zoe’s water will break, which is notable only in that it signals the end of the movie is near. Or so we hope.

With no help from the idiotic script, O’Loughlin actually manages to emerge from this mess unscathed. Give the guy another shot at a series, and he’ll be fine (especially if he’s allowed to take his shirt off).

But Lopez’s performance is shallow and sitcom-ish, full of winky tics and cutesy giggles. She has a few funny moments of horror while witnessing the living-room birth, but the laughs in “The Back-up Plan” are as rare as a good romantic comedy.

If only some screenwriter could give birth to such a thing.