Tina Fey, left, and Amy Poehler star in the mommy

“Baby Mama” is a pleasantly predictable new wrinkle on the “moving to the mommy track” comedy, which “Baby Boom” launched lo those many years ago. It’s about surrogacy, or as the annoying entrepreneur-zealot (Sigourney Weaver) who hooks up fertile women (“gestation assistants”) with those who can’t have kids puts it, “outsourcing” pregnancy.

And if this comedy filled with “Saturday Night Live” alumni isn’t a laugh riot, well, when was the last time anything producer Lorne Michaels stuck his name on a hoot?

Tina Fey stars as Kate, 37 years old and hearing her biological clock’s alarm go off.

“Katy’s coming out of the mommy closet,” teases her sister (Maura Tierney), who already has kids.

But Kate’s been career-oriented, tossing over men who want to settle down so that she could rise to vice president of Round Earth foods, a trendy organic supermarket chain. Now, working for the goofy guru in charge (Steve Martin, channeling every surfing, New Age pony-tailed corporate cliche in film history) isn’t enough. Kate wants kids.

Her “baby mania” scares guys off. And her ob-gyn tells her that the reason her designer-fertilized eggs won’t take is simple.

“I just don’t like your uterus.”

So she coughs up $100,000 to Weaver’s entrepreneur and contracts with Angie (Amy Poehler of “SNL”) to carry her egg to term.

Poehler presents Angie as a long-lost Spears sibling, a trashy, uneducated lout who says she “never has trouble” getting pregnant. Only she has no kids (figure it out). She has a lazy jerk “common law” husband (Dax Shepard, letting it all hang out). They figure to cash in on the one life-skill Angie seems to possess. Until they split up.

So “American Idol”-obsessed, junk-food eating, chain-smoking, gum-snapping Angie winds up living with uptight, micro-managing, baby-book-reading Kate in Kate’s posh Philly apartment waiting for the blessed event.

Fireworks, right? Well, not exactly.

“Baby Mama” has its moments, but it is somewhat less than the sum of its parts. Romany Malco of “40-Year Old Virgin” may be playing a cliche, the street-wise doorman. But he’s a funny one. Shepard and Weaver score big laughs. Martin’s character is more a funny idea than hilariously executed. Greg Kinnear, playing almost exactly the same guy he trotted out for “Feast of Love,” shows how easy his charm wears when he doesn’t have to tear-up or carry the movie (he’s Kate’s new love interest).

But Fey and Poehler, despite their “SNL” years, don’t really click. They don’t challenge each other enough to push their characters into the realm of outrageous. It’s as if they suppress each other’s performances. Angie’s true trashiness only shines when she shares a scene with Shepard.

Fey is pretty much a supporting player, a straight woman in her own movie, another version of her “30 Rock” role. Kate is snobby, but sweet. The “SNL” vet writer-director of “Baby Mama” (Michael McCullers) gives her a nice moment in an elevator, offering her finger to an infant and then smelling its hair. But Fey is more real than funny, and her many scenes with Poehler wear that label, too.

Still, there are a couple of minor surprises and enough laughs and truisms floating around the supporting cast to make this an easy comedy to watch. But the few gratuitous moments of profanity and trips to the toilet don’t give it “edge.” Lorne Michaels rubbed that off most of his troops years ago.