The predicament of unwed teenage mothers is not supposed to fill the populace with warm, fuzzy feelings. But first-time screenwriter Diablo Cody has written a new role model for teenage girls — one who happens to get pregnant at 16 after a one-night stand.

A lead character that lacks the most basic of human concerns can often devolve into misanthropy, but sometimes — as with the characters of Carrie Bradshaw and Samantha Jones in HBO’s “Sex and the City” — figments of the imagination can provide cheeky entertainment and more than a few life lessons. With “Juno,” directed by Jason Reitman, Ms. Cody has complicated the typical farce of high school life depicted in such films as “Clueless,” “Heathers,” and — her own inspiration — “Ghost World” by entangling the dilemmas involved in welcoming a new life just as the adolescent life is beginning.

Young Juno MacGuff (Ellen Page) has a hefty chip on her shoulder that has helped her navigate the pitfalls of high school and teenage angst. When she finds she has become pregnant as the result of a one-night stand with her best friend, Bleeker (Michael Cera), she bypasses the normal concerns about her predicament and calls up a clinic to “procure a hasty abortion.” Some mixture of disdain and guilt after encountering an anti-abortion classmate leads her to reconsider. Juno decides instead to give the baby up for adoption.

Flippant and detached about her difficult situation, Juno is forced to deal with aspects of adulthood that she deems “beyond her maturity level.” But the overly glib approach to life that helped her find her way through the halls of Dancing Elk High won’t do much to protect her this time.

With ample wit and intellect, Juno stumbles through the steps to adulthood, learning that defensive humor is not capable of disarming all the difficulties of adult life. Though she treats most adults with disdain, Juno begrudgingly learns that there are some things she doesn’t yet understand. Of course, it doesn’t help that the adults in her life seem to be on the same course. Her father (J.K. Simmons) possesses some supportive and biting characteristics, while her oppressively nail-obsessed stepmother (Allison Janney) reserves her nurturing instincts for when the need arises. And the perfect couple Juno has chosen to raise her child (Jennifer Garner and Jason Bateman) clearly have their own problems.

Though Mr. Reitman’s film sometimes takes a rudimentary approach to these issues, the director saves it from superficiality with a deep interest in capturing the heart of his main character.

The 30-year-old Mr. Reitman, whose last film, “Thank You for Smoking,” offered a sharp satire set in the boardrooms of Big Tobacco, confidently captures the one-step-forward, two-steps-back progression of teenage maturity better than most directors. Mr. Reitman was a teenager as recently as the first Clinton administration, and there is an obvious empathy present in his light-handed approach to a somber subject. The mood is aided by the occasional chipper cartoon and a jangling, hipster-friendly Wes Anderson-style soundtrack featuring songs by Kimya Dawson, Buddy Holly, Cat Power, Belle & Sebastian, Sonic Youth, and the Kinks.

It helps that Ms. Cody’s script crackles with life, humor, and emotion. The self-effacing teenage misanthrope is not new to the silver screen, but Ms. Cody manages to maintain the vibrancy and confusion of pubescent life throughout this complicated situation. That, combined with Ms. Page’s intricate portrayal, makes the character of Juno one of the smartest, funniest teenagers in film in a long time. Imbued with a startlingly self-aware humor that is lacking in most young screen characters, Juno is an antihero for a generation raised on MySpace and plasticized reality television. When she is told one day that her parents might be worried about her, she coolly responds, “I’m already pregnant. So what other kinds of shenanigans can I get into?”

She may be susceptible to overeager machine-gun dialogue, but “Juno” is saved from the glib dredges of misanthropy by an irrepressible compassion. Young Juno may not always make the right choices, but she earnestly tries her best to do right. When she nervously informs her father about her predicament, he is understandably disappointed, and tells her that he always thought she was the kind of girl who “knows when to say ‘no.'” Crestfallen, she responds, “I don’t really know what kind of girl I am.”

The beauty of “Juno” resides in the complicated journey on which it sets its heroine to figure that out. Unguided by any overarching worldview or by reliable authority figures, she often stumbles and falls. But as the film posits, sometimes it’s the journey that matters most.

mkeane@nysun.com