As Kate Holbrook, the unfashionably infertile heroine of "Baby Mama," Tina Fey embodies the reproductive flip-side of "Juno," "Knocked Up," "Waitress," et al. Though she wants a baby in the worst way, Kate is facing the cold, hard truth of being a would-be mother in her late 30s: It's hard to conceive.

"I just don't like your uterus," her fertility specialist (John Hodgman) quips. "It's T-shaped."

Kate's story – single, unfulfilled career woman, hungry for motherhood – has all the makings of a good serio-comic baby saga, but Fey and writer-director Michael McCullers aren't too keen on the "serio" part. Instead, they pony up a reliably lightweight buddy movie, pairing Fey with longtime "SNL" cohort Amy Poehler in an amnion of "Odd Couple"-style situational humor that – while full of modest laughs – fails to carry the movie to full, emotionally-rounded term.

The movie has some hilariously mischievous moments, particularly when McCullers (the "Undercover Brother" screenwriter, here making his directing debut) exploits Kate's ordeal to satirize the phenomenon of modern-day reproductive baristas. When Kate – vice president of a Philly-based organic foods company – struts out of a sperm-donor facility with a no-drip canteen of baby-batter, the implication is clear: She might as well be lugging a soy latte from Starbucks.

When the sperm-donor method fails to produce results, Kate turns to a surrogate-mother headhunter (Sigourney Weaver, funny as a shameless "lifestyle" shill) and pays $100,000 to implant her inseminated eggs in Angie (Poehler), a sweet but trashy wannabe clothes designer with an opportunistic common-law husband (Dax Shepard) cut from the same ribbed-undershirt cloth as Kevin Federline.

Though imperfectly cast as the young, semi-literate, child-bearing type, Poehler brings her substantial comic focus to bear on Angie, and owns the role. And when Angie leaves her husband and moves in with the control-freaky Kate, the two comediennes are only too eager to de-mothball their finely-honed "SNL" rapport.

It's Kate's pre-natal vitamins vs. Angie's Pringles, with karaoke duets of "Girls Just Want to Have Fun" to hasten the bonding. (Consequently, Greg Kinnear is an afterthought as Kate's reformed-lawyer love interest, though Steve Martin and "Weeds" regular Romany Malco chip in sharp supporting performances as her spacey new-age boss and wise-cracking doorman, respectively.)

In her first bonafide leading-lady role, Fey proves something of an enigma. With her bespectacled good-looks, city-gal smarts and endearingly uptight energy, she recalls a "Baby Boom"-era Diane Keaton, but with a nagging self-consciousness that prevents Kate from assuming truly human form. She just doesn't pop, in more ways than one.

“Baby Mama”
Stars: Tina Fey, Amy Poehler, Greg Kinnear, Dax Shepard
Behind the scenes: Written and directed Michael McCullers
Rating: PG-13 (crude and sexual humor, language and a drug reference)
Running time: 1 hour, 36 minutes
Grade: B-