Is any premise more overused in movies and television today than the one about a self-centered person who’s changed for the better by suddenly becoming responsible for a child? Often it takes the form of an individual who learns there’s a kid he didn’t know he had or abruptly gets custody of (among recent entries, see “The Switch” and the sitcom “Raising Hope”), or a couple in similar circumstances (see “Life Unexpected”). You can trace the idea back to silent movie days, and more recently even Adam Sandler got into the act with “Big Daddy.”

But the grimmest variant of the scenario involves the deaths of the real parents, which hangs like a cloud over any effort to make the aftermath into a sprightly, frothy affair. The old TV show “Bachelor Father” suffered from the syndrome, and so did Garry Marshall’s “Raising Helen.” Given this history, one might think that filmmakers might be deterred from embracing the premise yet again. But it resurfaces once more in the flatly-titled “Life As We Know It,” which should really be called “Life As Formulaic Movies Show It.” As befits a feature coming from makers who’ve made their mark in television, it’s nothing more than a second-rate sitcom stretched to unconscionable length by, among other devices, probably the largest number of musical montages in the history of the genre. And while some viewers may think it sweet, it’s actually kind of creepy—especially when one of the new parents wonders aloud whether her deceased friends “planned this,” referring to getting her and the guy she’s been paired with together, but by extension suggesting the parents’ deaths and their infant daughter being left utterly alone.

Whatever the case, the unlikely child-rearers are Holly Berenson (Katherine Heigl), the uptight, disciplined owner of a small bakery, and Eric Messer (Josh Duhamel), an assistant director for the television broadcasts of the Atlanta Hawks who is, of course, a womanizing adolescent in a grown man’s hunky body. They’re the best friends of Peter and Alison Novak (Hayes MaCarthur and Christina Hendricks), the parents of one-year old Sophie (Alexis, Brynn and Brooke Clagett). But when the Novaks set up Holly and Eric on a blind date, it ended up terribly, and they’ve barely tolerated each other ever since. So why the couple should have legally named Holly and Eric as Sophie’s joint guardians in the event of their death is puzzling. And why they never even discussed the matter with their pals is even more absurd and makes them, true to tell, pretty irresponsible parents.

To add to the implausibility of it all, the lawyers advise Holly and Eric to move into the Novak house to care for Sophie together, and the social worker assigned to their case impresses them with the importance of not developing a romantic relationship. Of course they do, in a plot thread that will come as no surprise to anybody and comes off as a dismal retread of the “opposites attract” motif of the old Tracy-Hepburn pictures. Poor Josh Lucas is stuck with the thankless Ralph Bellamy role, the third wheel in the romance, as Sophie’s recently-divorced pediatrician (who just happens to have met Holly “cute” before as a patron at her bakery).

That part of the plot—which includes, if you can believe it, another version of that old “running to the airport to catch the person you’ve just sent away” business that we’ve just seen for the umpteenth time in “Going the Distance”—is complemented, if you can call it that, by the inevitable section dealing with how these two non-baby people learn to change diapers and induce the kid to eat. The only good thing you can say about this part of the picture is that since the child is a girl, we’re spared the scene in which it sends a urine spray into the face of whoever’s wrongly situated during the diaper change.

There’s nothing remotely charming or funny in all this, and the leads don’t help. Heigl, as is becoming her habit, mugs too much, and Duhamel plays everything in pure sitcom style, perhaps simply satisfied with a role that flatters him with a sequence in which all the neighborhood women (and the requisite gay dude) drool after him when he goes jogging. Of the supporting players, Lucas certainly suffers most, but Melissa McCarthy earns no prize as one of the neighbors, coming across as a bargain-basement Edie McClurg. Director Greg Berlanti, a television guy himself (along with his scripters) except for 2000’s indie comedy “Broken Hearts Club,” doesn’t help any of them with a heavy-handed approach that suggests he doesn’t know the difference between the needs of the big screen as opposed to the small one. The technical contributions are adequate, but as usual in these kinds of movies, Blake Neely’s score can be gratingly chirpy.

“Life As We Know It” feels like one of Rob Reiner’s or Ron Howard’s saccharine misfires. Those of you who should avoid items with a high sugar content are hereby warned.