The “odd couple” of a yuppie and her low-rent surrogate mother is not the only clash of sensibilities on display in the new comedy “Baby Mama,” starring Tina Fey and Amy Poehler. The broad laughs and stock scenarios telegraphed by the movie’s standard-issue buddy-movie posters don’t entirely drown out the verve of two razor-sharp performers and the flashes of sharp, absurdist writing. Indeed, the quicker-footed work peeping through makes this an ironically appropriate opening film for the Tribeca Film Festival and its big-tent, mixed-bag scheme.

“Baby Mama,” which made its gala premiere Wednesday and opens nationally today, gives deserved center stage to two “Saturday Night Live” associates. Ms. Fey, the show’s former head writer and Weekend Update anchor, plays Kate, a natural-foods company VP spooked by her biological clock. Ms. Fey is (mostly) the straight woman to current “SNL”er (and Update anchor) Ms. Poehler, as Angie, a rough-around-the-edges Philly girl looking to turn a buck through childbearing (having “discontinued high school” years ago).

After several spats with her trashy boyfriend (Dax Shepard), Angie moves in with Kate, leading to pre-partum disputes and cross-class misunderstanding. Kate frets over her surrogate’s junk-food habit and penchant for sticking gum under a “reclaimed barnwood” table. Angie chafes under the vested concern of her host and spends days caterwauling on the video game “Karaoke Revolution.” The two are at entertaining ease with one another, though Ms. Poehler steals the show — alternately spiky, squirrelly, sulky, and relaxed.

But, alas, bigger, noisier comedy gears are grinding as well. Romance is kindled with a juice-joint owner (Greg Kinnear), whom Kate’s gentrifying company is squeezing out, while her good-hearted black doorman (Romany Malco) gives timely advice and cracks wise. (On the surrogacy scheme: “Ask any black man in Philadelphia, she’s a ‘baby mama.'”) Steve Martin impressively breathes life into a ’90s-era parody as Kate’s eco-snob boss, a ponytailed millionaire with an eye for profits, name-dropping, and ludicrous koans (exit line: “Time is love”). Sigourney Weaver also turns up as the maddeningly fertile head of the surrogate clinic.

As with Mr. Martin’s dialogue, the sharper lines can perk up scenes, especially when they flirt with absurdity. In one recurring gag, Angie is baffled by the automatic locks on Kate’s car and, generally, both comics show strong timing with their occasional sarcastic zingers. After all, Ms. Fey and Ms. Poehler (who is also a founding member of the Upright Citizens Brigade comedy troupe) honed their chops at ImprovOlympic in Chicago in the ’90s. On e gets the sense they could generate endlessly delightful tomfoolery, such as a sketch-like courtroom scene (don’t ask) in which Angie addresses the judge with “Aye, aye, Captain.”

But mostly, “Baby Mama” trucks along toward resolution, with riffs on such upper-middle-class customs as organic food and gentrification — bits that are hard to make fresh. Ms. Fey bears most of the burden of these jokes and, blunted somewhat, often demonstrates her skill at taking insults. But the sweet, slightly nerdy side that’s part of her appeal on the terrific NBC sitcom “30 Rock” is mostly replaced here by the more commonplace lameness of Kate’s class tics (though, refreshingly, she is just as good at doing her job).

In any case, the two stars can’t recoup this twist (written and directed by Michael McCullers) on the usually male-indulgent genre of the reproductive comedy of errors, though Ms. Poehler, especially, works up deliciously askew pockets of energy. But the upshot, and upside, of “Baby Mama” is that Ms. Poehler and Ms. Fey should pair up for another movie that gives freer rein to their talents.