Here is an AP story on Diablo Cody, the much talked-about screenwriter of Juno. The article explains that the film comes from personal experience, that Juno is a fairly autobiographical character, and the teen pregnancy plot comes from a high school acquaintance.
This is effective evidence supporting one position that I have about Juno - that it's a film about a late-wave Generation X teen-ager veiled in the slang and accoutrements of the young people of today. A comedy about teen pregnancy involving a hip teenager who namedrops Iggy Pop and whose goal is to play in an indie band seems much more suited for 1994 or so. That happens to be the year that the 29-year-old Brook Busey-Hunt (aka Diablo Cody) was 16 years old. This is like a Baby Boomer penning a script in 1986 about a deeply moralistic and politically concerned teen-ager raising the social consciousness of her classmates and getting them to participate in a world peace march. I seem to vaguely recall some plots like that floating around the culture of the time.
That doesn't make it a bad movie. It does make it a memoir.