Still-single career woman Kate Holbrook (Tina Fey) is screwing up another of her increasingly desperate dinner dates. "I'll be the oldest mom in day school," she tells the startled guy across the table. "I want a baby now. I'm 37." Cue shot of guy beating a retreat out the restaurant door.

A couple of plot developments later, trailer-bait doofette Angie Ostrowiski (Amy Poehler), whose hired womb will soon be home to Kate's artificially fertilized eggs, is being infused with hospital-grade drugs for the implantation. She's loving it. "This is good," Angie says, taking off. "What's the street name for this?"

These two scenes suggest the several strengths and a minor weakness of "Baby Mama," a movie that may be rooted in the familiar bog of TV sketch comedy, but which blossoms, by way of sharp writing and some inspired performances, into a picture that's funnier than you might expect, and occasionally hilarious.

Any movie about reproductive anxiety and surrogate motherhood is a chick flick by definition. But a good chick flick — "Moonstruck," say, or "Bridget Jones's Diary" — is always a good guy flick, too. "Baby Mama" isn't a lecture about sex-role burdens; it's really about social worlds in collision. Kate, a top exec with a Philadelphia-based health-food chain called Round Earth (a name to savor), has everything except what's beginning to seem the most important thing. "I just woke up one day," she tells a friend, "and I felt like every baby on the street was staring at me." She sets out on a quest for motherhood, but learns that pregnancy is not in her particular cards. Deciding to try in vitro fertilization, she winds up at an agency that specializes in recruiting surrogate mothers. After handing over a hundred grand, she is soon hooked up, most unpromisingly, with Angie.

Angie has been steered into this line of work by her layabout boyfriend, Carl (Dax Shepard). Carl is a classic of his kind (the sort of guy who has a pet iguana called Hellboy), but Angie has been glued to him since "the summer after I discontinued high school." When they suddenly split up, Angie, having no place else to go, decides to move into Kate's swank apartment. This could only be a good idea in a movie, of course. Kate lives in an idiotically upscale world in which baby strollers come not just with iPod holders, but with airbags, too. Angie, on the other hand, is into aura-reading and junk food. As you'd hope — as in fact must by comedy law be the case — chaos soon reigns.

Fey, in full, gorgeous fluster, and Poehler, a master of comic delirium, are nicely paired; and the script, by writer-director Michael McCullers (who toiled on the "Austin Powers" movies), gives them a sizable number of killer lines to work with. Greg Kinnear, affable as always, is on hand to offer Kate romantic hope as the owner of a struggling juice shop; and the movie also has the immeasurable benefit of two small, scene-jacking performances by Sigourney Weaver and Steve Martin. Weaver plays the insufferably smug owner of the surrogate agency, a woman in her fifties who has managed to become pregnant herself with no artificial assistance. ("You should see my uterus," she tells the despondent Kate.) And Martin, heavier here, and more satirically zestful than he's been in years, is Kate's ponytailed boss, Barry, a health-food mogul and new-age buffoon ("I was swimming with the dolphins this morning in Costa Rica ..."). Add on the invaluable Romany Malco as a nuts-and-bolts love man ("I don't have relationships, I have relations"), and you have a comedy that delivers more smart laughs than its genre might seem to promise. Guys, please be advised.

Check out everything we've got on "Baby Mama."

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