In a gene pool teeming with runt sequels and in-bred ideas, Hollywood comedy desperately needs fertile minds like Judd Apatow to go forth and multiply.

After producing Will Ferrell hits Anchorman and Talladega Nights and making a star of Steve Carell with his directing debut The 40 Year Old Virgin, Apatow has fathered another bundle of comic joy.

Virgin sidekick Rogen plays Ben, a professional slacker idling his life away with his bonged-out housemates. The boys intend to get rich quick through a nude-celebrity website but put most of their energies into spliffs, musing on pop culture and taking the proverbial out of their pal’s beard.

Unlike Ben, however, Alison (Heigl) is going places. She’s out to celebrate her promotion from floor manager to presenter on TV channel E!’s hottest show.

Thanks to the magic of alcohol, the pair hit it off at a nightclub and consummate the liaison with a moment of unprotected madness. Next morning, they go their separate ways.

Two months later, a mortified Alison drops a bouncing baby bombshell on Ben. Like it or not, they’re both on board for a seven-month rollercoaster of uncertainty, introspection and the encroaching dread of responsibility.

The beauty of Knocked Up is that Apatow and his cast bring warmth and heart to each and every character while exposing their shortcomings with brutal honesty.

Nature dictates that guys are jerks and girls are irrational, but any misfortunes they suffer, they bring upon themselves.

As Alison’s sister Debbie and her acerbic husband Pete, Leslie Mann (aka Mrs Apatow) and the ever-watchable Paul Rudd are terrific, perfectly nailing the ‘is-this-it?’ frustrations of a married couple for whom the honeymoon is well and truly over.

Even the smallest roles make their mark. We’re treated to the philosophy of a night club doorman, the David Brent-like sourness of a female TV exec, and the ice-breaking styles of LA’s gynaecologists ("You do look like your sister!" observes one from his unique vantage point).

With the labour lasting over two hours, a few stretch marks are inevitable. Ben and Pete’s trip to Vegas and a nasty infection in the boys’ house get laughs but really belong in the DVD’s deleted scenes.

It’s also odd that, sartorially speaking, Heigl should only be as bold as bras in a role demanding zero coyness and umpteen sex scenes.

But these are minor complications when the gags, ahem, come as thick and fast as this. When’s the next one due?

Elliott Noble