I'm not sure that this story about a teenage girl who gets knocked up, considers an abortion but decides instead to give "the thing" to a well-lined middle-class couple is quite as good as the American critics would have us believe.

Certainly it contains a witty and offbeat screenplay from Diablo Cody (who has also written a book about her year as an unlikely stripper which must soon be filmed). And the central performance from the elfin Ellen Page is remarkably fresh and controlled.

But often the lines seem unlikely to spring from teenage sources and have the sophistication of a writer instead.

Added to that, a certain glibness pervades, as if the writer and director (Jason Reitman, of Thank You for Not Smoking) has finally opened the book marked "adolescent mysteries" for parents who would otherwise have no idea what their children do and think.

Perhaps, of course, Cody and Reitman have done just that, and it can certainly be said that the film comes properly alive when Juno starts to get to know the hopeful recipients of her baby (Jennifer Garner and Jason Bateman).

Despite having gone for a private adoption, she begins to feel the need to share the whole experience with people who care. In other words, she has to grow up and at least partially desert her scatty schoolfriends.

The more she gets to know the parents-to-be, the more she understands the nature of her choice.

So despite the superficiality of the opening scenes, the movie finally deepens, without sentimentality, into something more significant than sharp comedy. That's when the praise it has received is really deserved.