Actress Anne Hathaway turned down the lead role in romantic comedy Knocked Up because she deemed the movie "too radical", according to the film's star Katherine Heigl.

The Grey's Anatomy beauty landed the role of Alison Scott opposite Seth Rogen after Hathaway, who was the original choice of director Judd Apatow, refused to take the part.

But Heigl was only too happy to be cast - and was even delighted when audiences were shocked by graphic footage of a baby's head crowning during the movie's birthing scene.

The 30-year-old says, "I think Anne found the film a little too radical especially with the shot at the end!"

"Seeing a baby's head poking through was a little strong but the reaction of the audience at that point was so extreme. They just screamed - especially the men!"