Kate (Tina Fey) is romanced by juice bar owner Rob (Greg Kinnear) in Baby Mama.

Universal studios

Pregnant with complex twists that give birth to surprising complications, the often hilarious Baby Mama breaches its high-concept idea of a 37-year-old woman who hires a surrogate mother to carry her baby to term.

If you feared that Baby Mama was going to be a one-joke idea, rest easy. Writer-director and former Saturday Night Live alumnus Michael McCullers, who wrote two Austin Powers movies, makes an auspicious directing debut with his rollickingly fast and funny (and occasionally touching) script. With the expert comic talents of fellow Saturday Night Live alums Amy Poehler and Tiny Fey (who went on to create and star in 30 Rock), McCullers gives his film a welcome edginess with material that dares to broach everything (although gently) from race relations to the cultural differences between the haves and the have nots.

McCullers has embellished his film with loads of clever subplots, pulled off by a cast of supporting players who have a razor sharp sense of comic timing: Sigourney Weaver as an over-the-hill Earth Mother; the unbilled Steve Martin as Fey’s insufferably self-satisfied boss, Dax Shepard as the surrogate mother’s low-life common-law husband; Romany Malco as a doorman who enlivens every scene with his pointed observations on the real world; Siobhan Fallon Hogan as the birthing teacher whose offbeat sense of pwonunciation dwops in Ws in words that have pwominent Rs.

Tensions inevitably rise to the boiling point between Fey’s Kate, a high-profile single career woman who has risen to become the vice president of an organic supermarket chain, and Poehler’s Angie, a wrong-side-of-the-tracks gal who pulls up to Kate’s swanky building in a junker driven by her common-law husband of 12 years. He’s the one who has prodded the reluctant Angie to become a surrogate mother strictly for the cash.

Despite their apparent background differences, Kate at first hopes for the best from Angie, despite her slovenly apartment and reluctance to eat organic foods and listen to soothing music. Kate has hired Angie through Weaver’s surrogate mother agency with all assurances that each “baby mama” has been screened thoroughly and so … not to worry. Hah! It’s not long into the enterprise, however, when Angie turns up on Kate’s doorstep in need of a place to stay. Although Kate at first sees her new housemate as a chance to refine Angie’s rough edges, it’s not long before the little annoyances become major bones of contention.

McCullers eventually drops a bombshell on the relationship that threatens to upend the whole project. But until then, there’s lots of funny back and forth between these two as Kate tries to improve Angie’s mind and the gum-chomping Angie tries to resist her every step of the way.

The movie’s clinical insemination scene becomes a hilarious hospital ballet with Diana Ross and Lionel Richie warbling “Endless Love” on the soundtrack. Later, Poehler makes the most of a scene in which the food-and-vitamin-obsessed Kate tries to get her to swallow a huge pill that looks like something you’d give a horse. Still later, Angie tries to get out of her own restrictive motherhood lifestyle while also loosening up Kate with a girls night. They put on sexy dresses and go to a hot nightclub, Angie all the while trying to sip forbidden alcoholic drinks behind Kate’s back.

Yet there’s a downside to all the hilarity as the ladies begin to get close and Angie begins to fret about the dark secret she’s harboring concerning Kate’s unborn child. Here the vast cultural differences between the two women really come into play as Baby Mama races headlong into unexpected and sometimes uncomfortable territory. But Kate and Angie have come too far to go back to where they started … and there is that baby to think of.

Puppyish Greg Kinnear turns up as the sweet-natured owner of a fruit juice emporium who begins to fall for Kate. But even with this character, who at first seems designed to offer an easy out to an increasingly complicated situation, McCullers inserts situations that threaten to pull him and Kate apart.

Still, the emphasis is on laughs and there are plenty of them. Weaver has a ball as a perpetually happy Lady Bountiful who equates surrogate motherhood with outsourcing and never allows even the direst events to upset her even-keeled life. Martin is subtly hilarious as Kate’s self-centered boss who is a font of New Age totems.

The supporting cast makes a strong backup for Poelher and Fey as they try to forge this most unusual relationship. In the end, Baby Mama really delivers.

****Baby Mama

Starring: Amy Poehler, Tina Fey, Greg Kinnear, Dax Shepard, Romany Malco, Sigourney Weaver, Steve Martin.

Rated: PG-13, contains crude and sexual humor, profanity, a drug reference, adult themes.

mjanuson@projo.com