“What To Expect When You’re Expecting” is what you might expect from a movie based on a popular pregnancy how-to manual: It’s about nine months long.

Five stories of expectant mothers intertwine: Jules (Cameron Diaz) is a fitness guru trying not to let pregnancy slow her down; Wendy (Elizabeth Banks) finds that her long-awaited pregnancy is more difficult than she anticipated; Holly (Jennifer Lopez) eagerly awaits an international adoption; Skyler (Brooklyn Decker), Wendy’s young stepmother-in-law, breezes through a twin pregnancy; and Rosie (Anna Kendrick), a young food-truck vendor, is shocked to find she’s expecting.

There are men here, too, but they don’t matter much — except during the movie’s frequent and sporadically funny cuts to a posse of dads at the playground, led by Chris Rock and a spectacularly bored-looking baby girl. (She really deserves more screen time; somebody get that kid an agent and a plotline).

All of these stories unfold with the expected accessories — vomiting, hormonal angst, impressively real-looking bellies and excellent hair all around — and everyone turns out, as expected for this sort of movie, just fine.

Meanwhile, the “What To Expect ” paperback lounges around in a contractually obligated fashion in the background, like a persistent house guest who’s being politely ignored.

Maybe if the characters had read the book, they wouldn’t have to talk about how surprising everything about pregnancy is. But then there wouldn’t be a movie.

The plot lines move along placidly, so there’s plenty of time to ponder extracurricular issues, such as why Elizabeth Banks made hormonal rages so funny on “30 Rock” (“Why does everything smell like onions!?!”) and so dull here, or why Jennifer Lopez appears to be the only woman alive who can have an elaborate emotional meltdown while maintaining perfectly glossed lips.

Rock has a few good moments, as does Decker’s sunny trophy wife, who’s experiencing a picture-perfect pregnancy (in the labor room, she sneezes and out comes a baby).

But mostly “What To Expect When You’re Expecting” feels like five mediocre romantic comedies uncomfortably squished together.

Don’t expect too much.