“What to Expect When You’re Expecting” is a “Valentines Day” take on impending parenthood. Assorted couples cope with pregnancies, planned and unplanned, adoption and the epic change that is coming to their lives.

It’s wafer-thin, but it has plenty of laughs — a lot of them involving pregnant women’s bodily functions, the rest coming from Chris Rock, who unloads lots of daddy-to-be wisdom on one prospective father. But what’s surprising is how touching this film from the director of “Waking Ned Devine” and “Nanny McPhee” manages to be. Kirk Jones and the screenwriters found real pathos in adapting the Heidi Murkoff self-help book, dubbed America’s “pregnancy bible.”

Elizabeth Banks plays Wendy, a self-help book author, a pregnancy “expert” who has never been able to get pregnant herself. Until now. She and hubby Gary (Ben Falcone) are all set to glow with the “angel’s kisses” of “this miracle.” And then her husband’s ex-racecar driver dad (Dennis Quaid) and his trophy bride (Brooklyn Decker) one-up them. Father and mother-in-law are expecting twins.

Anna Kendrick is the food-truck chef whose one-night tumble with a high school flame (Chace Crawford), also a food-truck cook, put her in a family way.

Cameron Diaz is a super-fit TV fitness guru newly pregnant with her “Celebrity Dance Factor” partner (Matthew Morrison of TV’s “Glee.”). Sure, she found out she was pregnant by throwing up on live TV. But she figures as fit as she is, she can do this pregnancy thing in her spare time.

And Jennifer Lopez and Rodrigo Santoro are buying the house and prepping for an adoption.  Santoro’s “Alex” is the guy his wife sends to a “dude’s group,” daddies with toddlers who trundle their kids through the parks of Los Angeles. And that’s where Daddy Chris Rock presides.

“Ready? There’s no such thing as READY,” Rock’s character, Vic, bellows. “You just jump on a moving train, and DIE.”

He and his crew make a lot of death jokes about what life is like after a baby enters the house. And cracks about the man’s loss of parity when there’s an infant in tow.

“Women pretty much control the baby universe,” so yeah, you’re buying a house, yeah, you’re deferring on every major decision regarding the baby. And yeah, babies “are where happiness goes to DIE.”

In montages, couples visit obstetricians or explain their state of mind to friends or colleagues. Couples bicker over matters big — circumcision, the baby’s name — and small. Couples struggle to endure, as couples, the strains of unplanned pregnancies.

Every so often, the “dude’s group” (Thomas Lennon is a member, and the very funny Joe Manganiello is the single, womanizing photographer-jock they idolize) gathers to dispense more warnings to Alex.

And then we return to Wendy, who has built a career out of romanticizing this experience, but who has no more clue about what she’s facing than her daft assistant (Australian comic Rebel Wilson, who is OUT there). If Rock is the voice of comic wisdom in “What to Expect,” Banks is its heart. She brings pathos and humor to a character who is hell-bent on loving this circle of life thing, until she’s overwhelmed.

Interestingly, the actresses involved in this movie all chose to play characters outside their own parenting experience. Lopez has children, and plays a woman who can’t. Banks, playing a woman determined to love pregnancy, had her baby through a surrogate. Kendrick and Diaz and model-turned-actress Decker aren’t moms — yet.

That doesn’t hurt the film, which is basically a light, superficial and frothy little romp through the pregnancy experience. It’s choppy and episodic, and funny — especially when Rock, a veteran dad in real life — is holding court. But the overarching message is both moving and amusing.

Expecting a baby? You have no idea what to expect.

MPAA Rating: PG-13 for crude and sexual content, thematic elements and language

Cast: Elizabeth Banks, Cameron Diaz, Chris Rock, Jennifer Lopez, Anna Kendrick, Dennis Quaid, Matthew Morrison, Crace Crawford, Brooklyn Decker

Credits: Directed by Kirk Jones, screenplay by Heather Hach and Shauna Cross, based on the Heather Murkoff book. A Lionsgate release. 

Running time: 1:38