Tina Fey didn't write "Baby Mama," though you'd be forgiven for walking into it and assuming she did. After all, her face appears prominently on the movie's ubiquitous posters, alongside that of co-star and former "Saturday Night Live" cast mate Amy Poehler.

The script actually comes from first-time director Michael McCullers, who previously wrote the second and third "Austin Powers" movies, but it could have used more of the mean girl. Mommy culture, with its capacity for smugness and solipsism, seems like a ripe topic for parody but "Baby Mama" approaches it with kid gloves.

The movie certainly has its zingers here and there and enough laughs scattered throughout to keep it bopping along in entertaining fashion - that is, until its ooey-gooey conclusion. in which every conflict works out way too neatly. The strongest moments, though, come from supporting players such as Steve Martin and Sigourney Weaver, despite the comic talents of its exceedingly capable stars.

Fey plays Kate Holbrook, the control-freak vice-president of a Philadelphia-based organic grocery store chain who finds herself in the position so many women do. Single at 37, after years of focusing on her career, she realizes she's desperate to have a baby. But when her gynecologist informs her that conception would be nearly impossible for her ("I just don't like your uterus," he says), she turns to Poehler's Angie Ostrowiski, an immature, junk food-eating, Red Bull-guzzling surrogate.

Predictable odd-couple highjinks ensue. But there are some surprises, too.

Weaver co-stars as the WASPishly named Chaffee Bicknell, who runs the surrogate agency even though she's freakishly capable of bearing her own children well past menopause. Her fertility is a fact that she condescendingly dangles over Kate, to great amusement; she views surrogate parenting as just another source of outsourcing.

Amazingly, Angie has passed all the background checks, even though she and her crass common-law husband, Carl (played broadly by Dax Shepard), come clunking into Kate's genteel life from the Philly suburbs blaring rap music from their junky car. (Kate's doorman and voice of reason, played hilariously by Romany Malco, warns her that baby mama drama surely lies ahead.)

When Angie leaves Carl, a philanderer and con artist, she has nowhere else to go so she moves in with Kate. Throughout all the obligatory gags about morning sickness and childproofed toilets - punctuated too frequently by the jaunty score from Fey's real-life husband, Jeff Richmond - it's obvious that Angie will help Kate loosen up and Kate will help Angie grow up. Still, Fey endears with her likable awkwardness and Poehler has just the right goofy energy and crazed look in her eyes to play opposite her.

While obsessing about all the usual stuff that preoccupies prospective moms, Kate also must juggle the demands of her New Agey boss, Martin's silver-ponytailed Barry, who makes clueless, pretentious asides like, "I was swimming this morning with the dolphins in Costa Rica." (He also rewards good work with five minutes of uninterrupted eye contact.) And she finds unexpected romance with Greg Kinnear's smart-alecky Rob, who runs the Super Fruity juice bar in the neighbourhood where Kate's company is building its flagship store.

Maura Tierney co-stars as Kate's overly fertile sister, with veteran character actress Holland Taylor as their mother, who disapproves of Kate's "alternative lifestyle." Siobhan Fallon Hogan also has a couple of standout scenes as a birthing instructor, whose speech impediment leads her to rhapsodize about the "gweat stwetch of dewivewy."

Of course, Fey and Poehler are front and centre. Watching them, you have the sense that, together, they're willing to go to any length for a great laugh - you just wish they'd been pushed into more challenging territory.

Two and a half stars out of four.