But within that affectionately messy sprawl lies a maturation, an effort to convey something deeper, more personal and more substantive. That goes beyond the casting of his real-life wife, Leslie Mann, as half of the couple in question, and the Apatow children, Maude and Iris, as the family’s daughters in this sort-of sequel to the 2007 hit “Knocked Up.” (That film’s stars, Seth Rogen and Katherine Heigl, are nowhere to be found, by the way. Which is fine.)

As writer and director, Apatow seems more interested in finding tough nuggets of truth than easy laughs. You can see a bit more clearly what he was aiming for with the ambitious failure of the serious, self-indulgent “Funny People” from 2009. Much of the banter between longtime Los Angeles marrieds Pete (Paul Rudd) and Debbie (Mann) can be very funny, but frequently it’s raw and painful as they have the kind of conversations about kids, finances and sex that might make many people in the audience feel an uncomfortable shiver of familiarity.

The film takes place during the three-week period when Pete and Debbie are both turning 40 (although Debbie likes to pretend she’s still 38). Birthday parties, fights about money, giddy getaways, school confrontations, bratty kid flare-ups and awkward attempts at reconciling with parents are among the many events that occur during this vulnerable time of transition. It’s like “Scenes From a Marriage,” but with poop jokes.

Although they would seem to enjoy a comfortable lifestyle, Pete wonders whether his passion project, the niche record label he founded, can survive. Debbie wonders whether either of the two young women who work at the cutesy boutique she owns is stealing from her. (One of them is played by Megan Fox in a surprisingly funny, disarmingly self-aware send-up of her own spectacular sensuality.) Pubescent daughter Sadie (a confident, expressive Maude Apatow) is wondering what the hell is wrong with her parents, while youngster Charlotte (an adorably goofy Iris Apatow) wonders what the hell is wrong with the big sister she used to know and love.

Fox’s performance is actually a great little microcosm of the film as a whole, and the expectations it upends. You think it’s this comfy depiction of a good-looking family that has it all, one that seems more than a little smug from Apatow, given the autobiographical elements that abound. But then he keeps poking holes in that facade until eventually he’s torn the whole thing down and the family itself appears to be on the brink of destruction.

Debbie and Pete search for answers — hey, maybe they can blame their parents’ mistakes for the mistakes they’re making now! — but eventually understand that they must look inward and support each other. This sounds like a pat, feel-good realization but it doesn’t come easily. These are people who rarely handle things well, from a discussion with the middle school principal to a visit with an estranged father. (The terrific supporting cast features Albert Brooks as Pete’s dad and John Lithgow as Debbie’s.) Rudd’s puppy-doggish, everyman likability still serves him well after all these years, and while Mann is humorously sharp and sometimes a bit too screechy, there’s also more depth to her performance than she’s been called upon to show before.

If “This Is 40″ feels a bit unsatisfying at the end, perhaps that’s because it’s meant as an interlude, a snapshot, a moment in time as the title suggests. It’s the feel-bad comedy of the holiday season — and that’s what makes it good.

“This Is 40,” a Universal Pictures release, is rated R for sexual content, crude humor, pervasive language and some drug material. Running time: 133 minutes. Three stars out of four.