It's the baby boomerang. You want a child, but can't conceive. Or you don't want one, and unintentionally get pregnant.

Or, as in Baby Mama - like Juno, Knocked Up and Waitress, a droll comedy where pregnancy is the situation - you can't carry to term, so you hire a surrogate mother who can.

These days, everyone outsources.

The farce stars Tina Fey as Kate, a buttoned-up Philadelphia business exec with baby fever, and her erstwhile Saturday Night Live partner Amy Poehler as Angie, an unzipped Joisey girl. (Except for a few exterior shots, Brooklyn doubles for Philadelphia.)

The Einstein Sisters of comedy physics, Fey and Poehler know that there is no absolute state of rest. The comic energy is all in motion, Fey's tension is released in Poehler's loose-limbed loopiness; Poehler's trespass of boundaries is kept in check by Fey's border-patrol persona.

On the evidence of their performances, Fey and Poehler are positioning themselves to become the next Dan Aykroyd and Eddie Murphy of movie comedy.

Admittedly, Michael McCullers' script is less Trading Places than Trading Spaces, with Kate bartering her bedroom for Angie's womb. Their transaction resolves the class tension between the careerist yuppie and the slacker surrogate, if incompletely.

Perfunctorily directed by McCullers, Baby Mama is best when skewering New Age entrepreneurs for what might be called Compassionate Capitalism. Steve Martin is sublime as Kate's boss, Barry, purveyor of organic food and Zen koans. Even slyer is Sigourney Weaver as the smooth-talking director of an exclusive surrogacy clinic who cheerfully explains that a surrogate is just a prenatal nanny.

In scenes more slapsticky than slapstick, McCullers explicitly frames the relationship between the driven yuppie and the trash-talking trailer tramp as a nonsexual romance. Embedded in his feather-light comedy is the insight that what with assisted-reproductive technology, the biological and surrogate mother form the New Age parental dyad.

To be sure, Kate and Angie have the obligatory male romantic interests, Greg Kinnear and Dax Shepard, who acquit themselves with dimpled charm. Funnier is Romany Malco (Knocked Up) as Oscar the doorman, who advances the plot and becomes Angie's confidant.

Some films have a third-act problem; Baby Mama has a third-trimester problem. McCullers takes all the loose plot strands and ties them in a neat knot and cuts it, as an obstetrician might a newborn's umbilicus. Still, I left with a smile.


Contact movie critic Carrie Rickey at 215-854-5402 or crickey@phillynews.com. Read her blog, Flickgrrl, at http://go.philly.com/flickgrrl/