aby Mama, Tina Fey's latest laugh-fest, is written, to a certain degree, with one contrivance too many. But that's Tina. Still, there’s something about her combination of smarts, goofiness and integrity that keeps us coming back again and again.

This time out, Fey is Kate Holbrook. Instead of a NBC TV executive, she's a buttoned-up Philadelphia executive with no social life, but a burning desire for a baby. She's 37, and hears her biological clock ticking. Her ponytailed boss at Round Earth Organic Market (Steve Martin in a rare and humorous appearance), who just promoted Kate, wants her to lead his plan to open new flagship store right in Philadelphia and Kate's thrilled… but what she really wants is a baby.

She soon finds herself in the lush offices of Chaffee Bicknell (a hilarious Sigourney Weaver), a woman of a certain age who runs a "center for surrogate parenting." One hundred thousand dollars later, Kate scores herself a "baby mama" in Angie Ostrowiski (former SNL cast mate Amy Poehler). This gum-snapping citizen of white trash is whom Kate hires to be her surrogate mother.

All seems good. Two weeks later, Angie calls Kate with the news: She's pregnant. But, complications quickly ensue six weeks later: Angie has left boyfriend Carl, a philanderer and con artist, and has nowhere else to go. So she moves in with Kate. From there, you can pretty much guess the rest.