Knocked Up fulfills the promise of writer-director Judd Apatow's first feature The 40-Year-Old Virgin, breathing new life into the graying cadaver of Hollywood romantic comedy. In fact, the film's few flaws arguably derive from Apatow not fully being able to secede from the hoary conventions of the genre he is reinventing.

Apatow's blend of forthrightness and naturalism blossomed and thrived (critically, not numerically) in the medium of television with his teen comedy dramas Freaks and Geeks and Undeclared, set in a high school and college respectively, which built cult followings with adolescent characters that seemed so dynamically, messily real . When Apatow moved to the big screen, he brought many of his distinctive young actors with him, developing a stable that includes Jason Segel, Martin Starr, and, most notably, Seth Rogen, who is also executive producer on Knocked Up.

In The 40-Year-Old Virgin, the eponymous frustrated middle-aged virgin, spectacularly embodied by Steve Carell, supplanted the frustrated teenage virgins of Apatow's TV series. As this recurring theme might suggest, Apatow is preoccupied with examining straight men, or rather boys, in their natural habitat, as they essentially grow up. He also debunks masculinity with a self-deprecating edge that manages to be simultaneously sophisticated and breathtakingly vulgar, poignant and hilarious.

For the uninitiated, the Apatow brand is a potent brew of high bar-setting improvised comic dialogue that manages to feel thoroughly fresh and true to life thanks to perfectly-pitched performances by actors that seem like everyday folks; and a naturalism that invests the characters with the complexities and foibles of real people struggling with problems. Many of the most joyous moments in an Apatow production come from weird characters, incidents or references that feel bracingly authentic despite rarely being seen on screen. Take for example the where-did-they-find-these-people supporting players (wacky dormmate Tina [Christina Payano] in Undeclared, foul-mouthed clerk Mooj [Gerry Bednob] in The 40-Year-Old Virgin, weird stoner girlfriend Jodi [Charlyne Yi] and passive-aggressive boss Jill [Kristen Wiig] in Knocked Up) thrown into the mix like Molotov cocktails. In tandem with his eye for great characters, Apatow's command of the way that people actually speak with each other and sensitivity to the more embarrassing details of being human exposes the poverty of the hackneyed dramatic conventions that we have come to expect from American studio filmmaking.

In Knocked Up, Rogen plays Ben, a twenty-something Vancouverite living illegally in California on cash from an insurance settlement. He spends his days smoking weed with his fellow Jewish doofus friends (and one goy), who are prone to such activities as farting bare-ass on each other's pillows - resulting in a pink-eye outbreak - and sparring both verbally and with sticks, ping-pong paddles and boxing gloves. Their grand plan: to make it rich from a website cataloguing female nudity in the movies, a project in which they have invested over fourteen months of their well-baked young lives. At a club one night Ben strikes up a conversation with the alluring E! Entertainment Television employee Alison (Katherine Heigl) - who is from the other side of the hygiene and ambition tracks - there celebrating her promotion to on-air interviewer. One thing leads to another, and one completely believable miscommunication over a condom later, Alison is throwing up in front of James Franco and discovering she is pregnant. The film follows the ups and downs of Ben and Alison's relationship from their uncomfortable post-fling negotiations to the baby's birth, keeping a subtle distance from most of the clichés. Apatow's subject might be musty - basically, how can the hedonistic pleasures of male camaraderie be happily balanced with the emasculating bonds of romantic commitment and the nuclear family - but the treatment is characteristically novel and natural, for the most part. With a welcome realism and rawness, it has enough surprises to make the phantoms of heteronormative family values and gender stereotyping that lurk behind the sexual candor go down easier.

The film mines its laughs and drama from the lifestyle clash between Ben's poor, stoner male homosocial clan and the conventionality and uptightness of Alison, who resides through the side door of older sister Debbie's (Leslie Mann) sprawling American dream, with its spontaneity-killing organizational calendar, and child molester and mercury paranoia. Even though Alison occasionally feels like a cipher - more reacting than acting - her relationship with Oprah-spouting Debbie, on her way to becoming an un-ironic carbon copy of her mother (played by Judith Light) provides some rich tensions and crackling sisterly dialogue, particularly about growing old and the foibles of men. Alison loves and depends on her older sister but fears becoming like her. It is quite striking that the character of Alison seems to inhabit a Hollywood romantic comedy far more than Ben does, and that her trust in the hype machine she works for - she even admits to loving Meg Ryan! - makes her just as much a dropout from reality as Ben and his buds.

While Rogen is a charming and unexpected romantic lead - a pudgy, furry mensch - you can sense him struggling a bit, but it is all for the greater good. Wet behind the ears compared to Carell's impeccable comic timing, Rogen's first starring role actually furthers the film's unaffected air even if he is just not as wildly funny. His low sense of self-worth is genuinely touching, especially when relating to Alison, whom he is keenly aware is out of his league. Paul Rudd meanwhile has graduated since The 40-Year-Old Virgin from acting the chum to reluctant role model, playing Pete, Debbie's husband. He initially seems cool and collected (if self-ironizing, speaking about his two daughters' love of bubbles: "Their smiling faces just point out your inability to enjoy anything") but as his friendship with Ben develops, we learn he feels confined and it becomes clear he is in no position to be guru to anyone. (Their boys' night out to Las Vegas begins with watching Cirque du Soleil while high on mushrooms and ends with a scene of abject, tearful masculine confession that signals the mens' darkest hour.)

Where Apatow really excels is in capturing the jockish one-upmanship of boys cracking wise, for example orchestrating a marathon running gag featuring a bet whereby if Ben's friend Martin (Starr) can go a year without shaving, the other guys will pay his rent. The down side of "The Dirty Man Competition": they will tease him mercilessly and if he caves and shaves, he has to pay all their rents. Thus throughout the film Martin is dubbed, among other things, shoe bomber Richard Reid, Serpico, Scorsese on coke, Robin Williams' knuckles and even Hassidic reggae artist Matisyahu. While the film ostensibly follows Alison's pregnancy, it also charts this greasy black beard's growth, offering a perverse parody of what it means to mature and become a man.

Through such brash riffing, Knocked Up works hard to earn its R rating, proffering enough imaginative scatology to keep the filthiest mind satisfied (including a memorable scene displaying Ben's phobic inability to have sex with a pregnant Alison for fear of his penis hammering the vulnerable fetus. In addition to a generalized frankness about the body's changes during pregnancy, one even gets a series of shock shots of crowning during Alison's delivery, giving young Jay (Jay Baruchel, star of Undeclared) a much-needed education in worldly matters. (Jay's maple-leaf tattoo, de rigueur for so many straight English-Canadian boys, testifies to the filmmakers' great attention to detail.)

The white elephant in the room, of course, is Alison's option of abortion - a choice that would threaten to terminate at mid-term the film's plot itself. Not surprisingly, Apatow treats the subject (if briefly) with comedy, outfitting Jay with a personal distaste for the practice that leads buddy Jonah (Jonah Hill) to mock-evasively refer to a procedure that rhymes with "shmashmortion." After a quick and brutal discussion between Alison and her power-suited mother, she decides to keep the baby and the subject is dropped. That matter decided, Ben struggles to accept responsibility and be a good father, provider and mate - somehow finding a great new job and apartment in a single montage scene. This predictable trajectory sits ill at ease with the candor and iconoclasm of its adventurous comic treatment. However, Apatow's accomplishment in revitalizing a tired genre is not diminished by his occasionally pat sexual politics; that he is carving a niche for riotously funny, adult comedies that embrace the colloquial over the canned within the Hollywood studio system is no mean feat.