Billed as the biggest, raunchiest comedy of the summer, it made perfect sense that Knocked Up was premiered on the largest screen in London and in front of a capacity crowd of nearly 2,000 people in various stages of deportment, excitement and flat-out inebriation. The movie, a savagely funny account of an unexpected pregnancy, was an inspired launch to Film Four’s open-air Summer Screen season at Somerset House.

For even at the final reel, with midnight fast approaching and a curiously cold wind whipping around the courtyard of this neo-classical eye-gouger, Knocked Up was still pummelling the audience with heavy hitting one-liners and irreverent sight gags.

The story, of course, is hardly revolutionary – boy meets girl, boy gets girl pregnant, and life as we know it is thrown into turmoil. And yet the blockbusting devil, as the writer-director Judd Apatow (creator of the comedy smash The 40 Year Old Virgin) clearly knows, is in the finely observed comedic details. Thus the boy, in this case, is a lazy stoner called Ben (Seth Rogen), and the girl is an aspiring careerist called Alison (Katherine Heigl). Ben is surrounded by a bestiary of equally enervated potheads with whom he banters gleefully about life, the Universe and the “breast and bush” shots in Trading Places.

Alison, meanwhile, lives with her sister Debbie (Leslie Mann) and her husband Pete (Paul Rudd), whose brittle marriage serves as a warning for Alison against the hell of lifelong commitment. Thus when Ben and Alison, after a drunken one-night stand, decide to proceed with her subsequent pregnancy, the stage is set for an epic clash of values and a slew of raw-knuckle comedy set-pieces – Ben and the heavily pregnant Alison, arguing in bed about Ben’s anxieties concerning pregnancy sex (he fears he’ll psychologically damage the baby) is typical of the movie’s innovative comedy candour.

But even as the plot proceeds, and themes develop, the movie reveals a softer, more reflective side. Here, it seems that all the protagonists are terrified of growing up, of moral responsibility, and ultimately of their own mortality. Mann’s Debbie, in particular, has a dark and strangely edgy scene at the velvet rope of an LA nightclub, where an impassive bouncer refuses her entry, cruelly, for being an “old bitch”.

And yes, Knocked Up can occasionally feel a bit, well, too male for its own good; it takes a certain chauvinistic chutzpah to make a movie about pregnancy that’s heavily reflected through male anxieties.

But this is possibly the point, and the balance that Apatow is seeking to redress. Either way, the crowds at Somerset House clearly weren’t complaining. The night ended with a mass distribution of official Knocked Up waterproof ponchos (on hand in case of the rain that never came, and soon to be found on eBay), and the very real sense that the season’s other Summer Screen gems, such as Rear Window and Rushmore, had just been given one impossibly tough act to emulate. On general release from August 24.

The Times is a media partner of Summer Screen 2007

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