Whenever people star in their own American Express commercials, they’re not just selling credit cards, but establishing themselves as products, too. Tina Fey’s recent Am Ex spot provides a perfect showcase for her now-established comedic persona as a harried but capable TV show-runner and mom. The ad extends Fey’s brand as the on-camera star of her sitcom 30 Rock, which itself fictionalizes her roles as 30 Rock’s creator and, before that, Saturday Night Live’s first female head writer.

In the same way that Jerry Seinfeld can’t really play characters, but is a genius at playing “Jerry Seinfeld,” Fey has cultivated a likable version of herself for performance. Fey’s characters come across like a more frantic Mary Richards or a flintier Ally McBeal. With her trademark glasses, Fey’s go-to character is defined more by her intelligence than many other funny female roles.

Fey’s 30 Rock weathered a shaky start to become possibly TV’s funniest sitcom. In the new comedy Baby Mama, Fey’s big-screen alter ego Kate Holbrook proves nearly identical to her 30 Rock alter ego, Liz Lemon, as another driven, self-deprecating careerist.

It’s easy to imagine Baby Mama’s surrogate pregnancy plot as on 30 Rock. It’s also painful to imagine, because the 30 Rock story would be snappy and insightful, whereas Baby Mama is leaden and obvious. Former SNL writer Michael McCullers wrote and directed Baby Mama, and though he tailors the film to Fey’s gifts, he doesn’t live up to Fey’s standards.

Baby Mama begins with the alarm ringing on Kate’s biological clock. A young vice president for a Whole Foods-esque organic food chain called Round Earth, Kate announces her wish to have a baby. When she sees multiple infants at a “Mommy and Me” yoga class, she seems close to salivating. Kate turns to surrogacy after facing difficulties with adoption as a single mother and realizing that she’s unable to conceive.

Chaffee Bicknell (Sigourney Weaver), owner of a boutique surrogate-parent company, pairs Kate with uncouth but fertile Angie Ostrowski (Amy Poehler). Baby Mama thus provides a cinematic reunion for Fey and Poehler, who co-anchored Saturday Night Live’s “Weekend Update” from 2004-2006. Unfortunately, the results prove more comparable to such unnecessary SNL movie spin-offs as Coneheads or It’s Pat.

When Angie has a falling out with her sleazy boyfriend Carl (Dax Shepard), she moves in with Kate. A predictable, female take on The Odd Couple for the 21st century follows, as Kate’s health-conscious, anal-retentive yuppie clashes with Angie’s working-class, junk-food junkie. At times the women seem to be kindred spirits. Hearing some particularly touchy-feely instructions at a birthing class, Kate says, “I think she wants me to rub olive oil on your taint.” “Couldn’t I just spray Pam down there?” Angie asks.

With her large, glaring eyes and shark-like smile, Poehler has a flair for playing insincere or scary women. She’s particularly funny in Baby Mama when caught in outrageous lies, and offers deft physical comedy when she can’t figure out childproof locks, or tries to steal other people’s drinks while clubbing with Kate.

But Baby Mama seems to have no sense of who Angie actually is, or would be in real life, as if actual blue-collar high school dropouts are unthinkably alien. Baby Mama offers the role some perfunctory credibility by making Angie aspire to fashion school, but avoids engaging potentially rewarding issues of female social roles.

Similarly, the script simply treats modern fertility trends as a laundry list of gags without considering the genuine emotions underneath them. A particularly mean, peculiar running gag involves Chaffee Bicknell conceiving a child “the old-fashioned way” in her 50s, and while Kate’s jealousy is understandable, her revulsion makes no sense.

Individually, some of Baby Mama’s jokes inspire laughs, such as Angie’s attempts to use a prosthetic belly called a “He-tus.” When McCullers tries to bring the plot threads together at a strained farcical baby shower and then a legal hearing, Baby Mama becomes painful to watch, like seeing loved ones trapped on Fear Factor. Maybe Fey can have it all one day, but Baby Mama suggests that her potential movie career will need a longer gestation.

BABY MAMA (1 ½ ) Directed by Michael McCullers • Starring Tina Fey and Amy Poehler • PG-13, 96 min • At Century Cinemas Del Monte Center, Maya Cinemas, Northridge Cinemas.