t's hard to imagine a more immaculately conceived or perfectly delivered comedy this year than Juno. From the striking first image of a girl staring quizzically at a discarded LazyBoy as she chugs juice, informing us in voiceover that "it started with a chair", we're putty in the hands of writer Diablo Cody as she crafts an astutely observed portrait of small town mores.

Every self-consciously clever line has been polished until it sings, but you'll be laughing too hard at Juno's vacillations to fully appreciate the genius of Cody's writing or the impeccable pacing of Jason Reitman's direction. Twenty-year-old Canadian actress Ellen Page is deservedly Oscar nominated for her portrayal of a laconic 16-year-old mother-to-be, who is refreshingly wise beyond her years, yet still believably naive when it comes to romance or the emotional strain of impending motherhood. She wears her heart and her nonchalance on her sleeve, contacting a clinic to organise a termination ("Hi, I'm just calling to procure a hasty abortion!") as if she were ordering pizza.

Juno MacGuff (Page) is understandably dismayed when a third pregnancy test comes back positive; the result of a moment of madness with misfit best friend Paulie (Michael Cera). Sharing the news with her shocked pal Leah (Olivia Thirlby), Juno nervously tells her parents Mac (J.K.Simmons) and Bren (Alison Janney) they are to be grandparents. She adds a second blow by revealing that she intends to give up the baby to a childless couple, Mark and Vanessa (Jason Bateman, Jennifer Garner). "We found them in PennySaver newspaper next to exotic birds," quips Juno.

As the plucky teen approaches full term, she develops an intense bond with Mark, who never achieved his lifelong dream of playing guitar in a rock band. Vanessa is compelled to reflect on her marriage while Juno considers whether Paulie is her soulmate, or as she so eloquently puts it, "the cheese to my macaroni".

Juno is a delirious comedy that takes a walk on the quirkier side of life, full of loveable, flawed characters we can root for, every tentative step of the way.

Page is delightful, capturing the maelstrom of emotions of a girl who has to grow up fast, and she generates a wonderful rapport with Simmons's father.

"I thought you were the kind of girl who knew when to say when," says Mac.

"I don't know what kind of girl I am," replies Juno sadly.

Janney is a hoot as a protective matriarch, who has the measure of everyone, including the ultrasound technician who thinks there is an art to squirting gel onto a belly.

"My five-year-old daughter could do that," quips Bren, "and she's not the brightest bulb in the tanning bed."

Supporting characters are sketched with similar affection, and Garner, Bateman, Cera and co respond with winning, multi-faceted performances.

In the words of the eponymous heroine, Reitman's film is "totally boss".