Ellen Page (left) and Olivia Thirlby

Thirty minutes into Juno , and it looks like the movie and its heroine are both in serious danger of being smothered by their own knowingness. A 16-year-old bundle of sass in a pint-sized package, Juno McGuff has responded to her unplanned pregnancy with all the guff befitting her name. As written in Diablo Cody’s debut script and performed by Ellen Page, Juno handles all crises with a string of pre-emptive verbal strikes—caustic one-liners that must have looked like immaculate repartee on Cody’s page, but play like the patter of someone who has been watching Ghost World and practicing Thora Birch’s part in front of the mirror. “Hi, I’m calling to procure a hasty abortion,” she fires into her plastic-hamburger phone after dialing the number for Women Now, a family-planning center she has selected “because they help women, now.”

The fact that abortion is mentioned—and that it arrives as Juno’s primary option—marks director Jason Reitman’s movie as the tough-minded comedic alternative to this summer’s Knocked Up . It’s certainly true that Cody’s strong-woman writing is a welcome corrective to the dude-centric vision of Judd Apatow’s crew. (It has become perfunctory for reviews of Juno to mention that Cody is a former stripper and phone-sex operator, which I believe makes her a neo-neo-neo-neo-feminist.) But while the women in Knocked Up could be sounding boards for Seth Rogen’s improvisation, they certainly were never as glib as Juno is in deciding to carry her baby to term and hand it over to a yuppie couple (Jennifer Garner and Jason Bateman) who have advertised their desire to adopt in the local PennySaver newspaper. (She dismisses abortion fairly swiftly, after disliking the atmosphere of the clinic and learning from a nearby protester that her baby “has fingernails.”) This setup is as improbable as it is self-consciously clever, and it threatens to derail Juno in the suburbs of a little town called Sundance.

All of which is to say that I walked into Juno expecting to love it (it’s an Arrested Development reunion, after all, what with Michael Cera playing Paulie Bleeker, the boy who very passively puts the bun in Juno’s oven), and halfway through was beginning to think I didn’t care for it very much. But the film soldiers on, and develops a gravity to match its gravidity. The movie is carried for a long stretch on the sheer conviction of Ellen Page’s performance; even when her lines sound false, the fear and uncertainty in her eyes is authentic and lovable. And then, much like that third pregnancy comedy of the year, Waitress , Juno turns on a heroine’s choice—between two men, who offer very different paths. Bateman plays the potential adoptive dad as a case of, well, arrested development, a man who still pines for his rock ’n’ roll glory days and takes the impending birth even less seriously than Juno does. Then there’s bewildered, loyal Bleeker—another inspired Cera creation, this time with the nervous smile erased by decency and concern.

As these two fellows reveal their true intentions, the movie’s patter slows to allow uncertainty and then warmth to slip in. Juno eventually describes herself (still sarcastically, but with a note of need) as “out dealing with things way beyond my maturity level,” and that confession rings in a third act that makes all the tough talk seem like the prelude to the discovery of real life. Juno —the girl and the movie—become wonderful when they admit how little they really know. PG-13.