Sometimes perfectly met expectations are a great thing. Case in point: Baby Mama, starring Tina Fey and Amy Poehler. Written and directed by Michael McCullers, a Saturday Night Live veteran and co-writer on the last two Austin Powers flicks, the comedy reunites the former “Weekend Update” co-anchors, and lets them butt heads and play off one another in exactly the mode, tone and fashion one could have predicted from the trailer, or a logline synopsis of the plot. And that’s enough, that’s what best serves this concept and story.

Kate Holbrook (Fey), a newly promoted vice president for Round Earth, an organic market chain, feels her biological clock ticking. Thirty-seven years old and unmarried, Kate has everything else in her life in perfect order: a nice apartment in a tony section of Philadelphia, a pony-tailed guru of a boss, Barry (Steve Martin), who values her, and of course all the financial security that brings. After trying to conceive through artificial insemination, and being told her chances are slim, Kate considers adoption, but is put off by the wait.

She then turns to a surrogacy center run by Chaffee Bicknell (Sigourney Weaver), which sets up a meeting with Angie Ostrowiski (Poehler) and her common-law husband Carl (Dax Shepard). Impressed by both Kate’s “aura” and her bank account, Angie agrees to be her surrogate. When Angie’s pregnancy test brings good news, Kate goes into nesting mode, but her well-organized plans are thrown into disarray when Angie shows up at her doorstep after a fight with Carl. All of Kate’s structure and patience is tested by the loudmouthed Angie, an under-educated caffeine addict, and the working definition of a “downmarket” voter who would surely be targeted by Hillary Clinton in a direct-mail campaign. Naturally, there’s also a potential love interest, in the form of juice shop owner Rob Ackerman (Greg Kinnear).

Fey’s innate likability and self-effacing demeanor keep Kate from ever becoming shrewish or overbearing. Poehler of course specializes in playing rude, screwy and chaotic, and here she plays the sort of woman who is just a rung-and-a-half, maybe two rungs up the socioeconomic ladder from one of the chicks from Rock of Love. The pair’s shared chemistry is what makes the movie work, and gives it some lift. Martin, meanwhile, is also a hoot, “transferring [his] success” by touching foreheads and gazing wordlessly into his workers’ eyes. Baby Mama doesn’t have a lot of complications, but it is witty enough, well paced, and tailored to its stars’ strengths and charms, which is all that it need be, truth told. (Universal, PG-13, 99 minutes)