aby Mama, Tina Fey's latest laugh-fest, is written, to a certain degree, with one contrivance too many. But that's Tina. Still, there’s something about her combination of smarts, goofiness and integrity that keeps us coming back again and again.

This time out, Fey is Kate Holbrook. Instead of a NBC TV executive, she's a buttoned-up Philadelphia executive with no social life, but a burning desire for a baby. She's 37, and hears her biological clock ticking. Her ponytailed boss at Round Earth Organic Market (Steve Martin in a rare and humorous appearance), who just promoted Kate, wants her to lead his plan to open new flagship store right in Philadelphia and Kate's thrilled… but what she really wants is a baby.

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While there are some surprises, for the most part, the film is as predictable as the title makes it sound.

She soon finds herself in the lush offices of Chaffee Bicknell (a hilarious Sigourney Weaver), a woman of a certain age who runs a "center for surrogate parenting." One hundred thousand dollars later, Kate scores herself a "baby mama" in Angie Ostrowiski (former SNL cast mate Amy Poehler). This gum-snapping citizen of white trash is whom Kate hires to be her surrogate mother.

All seems good. Two weeks later, Angie calls Kate with the news: She's pregnant. But, complications quickly ensue six weeks later: Angie has left boyfriend Carl, a philanderer and con artist, and has nowhere else to go. So she moves in with Kate. From there, you can pretty much guess the rest.

The two leads are funny as individuals, funny together, and they are fed enough funny lines to make "Baby Mama" work as breezy, lightweight entertainment, especially when they are both crammed in the same apartment.

Making his directing debut,writer/director Michael McCullers (who previously wrote the second and third "Austin Powers" movies) doesn’t help matters, shifting the tone of his screenplay between boorish bathroom humor, corny musical montages, and forced emotional confrontations without much concern for continuity.

Steve Martin is sublime as Kate's boss, Barry, purveyor of organic food and Zen koans. Even slyer is Weaver as the smooth-talking director of an exclusive surrogacy clinic who cheerfully explains that a surrogate is just a prenatal nanny. Funniest, though, is Romany Malco (Knocked Up) as Oscar the doorman, who becomes Angie's confidant.

Of course, Kate and Angie have the obligatory male romantic interests: Kate finds romance with Greg Kinnear's smart-alecky Rob, who runs a Super Fruity juice bar. Dax Shepard acquits himself with the usual dimpled charm as the bottom-feeding common-law husband. Fey and Poehler have considerable comic chemistry as the career woman and the surrogate mother, and Baby Mama keeps the laughs coming with bathroom humor, designer baby names, fertility-flaunting mamas and paternity tests - predictable odd-couple high jinks.

While there are some surprises, for the most part, the film is as predictable as that brief description makes it sound. It would have been great, however, for the film to be pushed into more challenging territory.