"Baby Mama" provides funnywomen Tina Fey (2004's "Mean Girls") and Amy Poehler (2007's "Blades of Glory") with their biggest and most dimensionalized feature film roles, to date, and that is a very good thing. They are fun to watch together and, having cut their teeth on "Saturday Night Live," are masters of improvisation. Both actresses avoid portraying their characters in broad strokes, choosing to allow the humor to come from more identifiable and human places. This gives "Baby Mama" a nice sense of levity—at least on occasion—but it is not enough to solve a screenplay by Michael McCullers (writer of the "Austin Powers" trilogy, also making his directorial debut) that treats its plot and conflicts with an alarmingly low level of intelligence. One suspects that had Tina Fey penned the film herself (as she did the aforementioned "Mean Girls" and TV's Emmy-winning "30 Rock"), it wouldn't have been half as conventional and plodding.

Kate Holbrook (Tina Fey) is a successful 37-year-old career woman with a cushy Philadelphia apartment and a top-shelf job as the vice president of the Round Earth Organic Market. A longtime singleton, the one thing Kate decides is sorely missing from her life is a baby. When she is told that her chances of conception are one in a million, Kate opts to hire a surrogate in the form of the directionless, borderline-white trash Angie Ostrowiski (Amy Poehler). After Angie breaks up with her no-good common law husband Carl (Dax Shepard), she ends up moving in with Kate. These two unlikely friends soon begin to forge a close relationship, but Angie is guiltily hiding a secret that could tear Kate's plans for parenthood apart.

A heartfelt comedy rather than a ballsy, skit-like slapstick, "Baby Mama" is unevenly paced but fairly consistently appealing for its first hour. The premise is one based upon Kate's initial desire for children, something that is unconvincingly developed and seemingly more of a gimmick to get the ball rolling. It's a flaw that can be overlooked, though, because of the strength of the two lead stars. The interplay between total opposites Kate and Angie is full of witty banter and onscreen chemistry—Tina Fey and Amy Poehler could go on to be a dynamic film duo—and extra points are earned by both performances rooting themselves in reality. Kate is a Type-A personality, but not witchy or overbearing, while Angie is free-wheeling, but not made to seem like a dim-witted buffoon.

The one element that does threaten to downgrade their IQs, however, is the secret that Angie is hiding about her pregnancy. Were she to simply speak the truth to Kate from the get-go, the film could have dealt with the intriguing ramifications of this truth rather than interminably dragging things out and culminating in a bunch of dramatic, sometimes mean-spirited confrontations and reconciliations. In turn, Kate's reaction to finally learning what's been going on is one of understandable anger, but also self-righteous name-calling that cuts short the sympathy the viewer feels for her. The movie's final third, which, no surprise, includes the hospital labor sequence, loses whatever comic momentum it has held up to this point and ultimately leads to a twist of an ending that is so convenient and overly precious it's not to be bought for a second. The last scenes aim to send the audience members out with an upbeat, nice-and-tidy wrap-up, but all it really leaves is an unpleasant syrupy taste in their mouths.

Backing up Tina Fey and Amy Poehler are a pair of strong supporting turns. Steve Martin (2006's "The Pink Panther") is a scene-stealer as Kate's calm, New Age-enlightened boss Barry, who awards people by giving them five minutes of uninterrupted eye contact. Meanwhile, Sigourney Weaver (2008's "Vantage Point") is hilarious as Chaffee Bicknell, the over-the-hill head of an offbeat surrogacy center who is still naturally conceiving babies of her own despite pushing sixty. Many age-centric jokes are thrown Weaver's way, and she is a terrific sport. Less successful are Greg Kinnear (2007's "Feast of Love"), saddled with the underwritten part of Kate's love interest, smoothie shop owner Rob, and Dax Shepard (2006's "Let's Go to Prison"), equally underwritten as Angie's troublemaking beau Carl.

As a star vehicle for Tina Fey's and Amy Poehler's respective talents, "Baby Mama" is passable, but less than it so clearly could have been. There are a handful of solid laughs—a sequence where Kate and Angie hit the club scene is a highlight—but there are more than a few duds in the mix, such as the multiple scenes in a Lamaze class that hinge totally on the coach's (Siobhan Fallon Hogan) speech impediment being funny. Nothing ever really comes of it, though, and those scenes hit a brick wall. The same could be said for the film itself, which strains for story conflicts and then solves them in lazy, predictable, and off-putting ways. Die-hard fans of Fey and Poehler need only apply; everyone else can safely sit the lukewarm "Baby Mama" out.