Two things you should know about the comedy "Baby Mama": The funniest parts were in the trailers, and the trailers weren't that funny.

There is a third thing you should know about the film, too, which may explain the disappointing failure - it is produced by Lorne Michaels. Michaels is the creator and producer of "Saturday Night Live," and he deserves a prominent place in the Comedy Hall of Fame for that accomplishment. But sketch comedy is what he does best, and he brings a sketch-comedy mentality to all his movies.

None more so than "Baby Mama," which plays out like a very long and very mediocre sketch on "SNL" from one of the bad years. This year, for instance.

The best thing the movie has going for it - the only thing - is Tina Fey. Fey starred in, and presumably wrote, the funniest "SNL" sketch of the year (a birth-control commercial parody), and she brings a sharp, contemporary and satiric eye to everything she writes, including "Mean Girls."

But she didn't write "Baby Mama." That chore went to Michael McCullers, who was an "SNL" writer for a year ('97-'98, with Colin Quinn, Cheri Oteri and Jim Breuer - one of the bad years). That's mistake No. 1. McCullers also directed. Mistake No. 2.

Fey stars as a fast-track career woman who, at 37, is suddenly desperate to have a baby. The usual methods don't work, so she takes the extraordinary step of hiring a surrogate mother, played by Amy Poehler.

At this point, every single step of the story is already determined. McCullers keeps with a slavish devotion to a standard formula and does not have the wit to disguise it. The second we see Poehler doing her white-trash bit, we know precisely what is going to happen. The second we see Greg Kinnear as a smoothie shop owner, we unquestionably know the plot's every supposed twist.

And no, it doesn't make it any funnier for being able to predict it.

In her first starring role, Fey proves that she can carry a film, or at least a weak film, though she never completely commands the screen. Behind her intelligent doe eyes, though, we see the despair, a recognition that the material is not up to her standards. Or at least we'd like to see it.

Poehler is Poehler, meaning she is fine at playing a broad character for a couple of minutes, but she has no business co-starring in a motion picture. Maura Tierney, who actually does have business in a motion picture, is wasted in a disposable role as Fey's sister, and far too much of the rest of the cast is peopled with "SNL's" lesser lights, along with Sigourney Weaver.

The one stand-out is Steve Martin, who plays Fey's pony-tailed, visionary, whacko boss. Even his few scenes aren't very funny (except a bit where he gives Fey five minutes of uninterrupted eye contact), but he imbues them with a comic force lacking in the rest of the film.

The basic idea behind "Baby Mama" isn't terrible, but as in all comedy it all comes down to the script. "Baby Mama" just isn't funny. It plays like a collection of scenes that were rejected from a better film.