Given everything that's happened since 'Knocked Up,' perhaps it would have been better if Anne Hathaway really had starred in the film instead of Katherine Heigl. Heigl has spoken out in the press a few times saying that she now feels like the part was sexist, and didn't agree with the way it portrayed women. Watching the movie - which is very funny - now doesn't feel the same because you know one of the main actors is hating every second of it.
Hathaway's objections had nothing to do with sexism - she just wasn't convinced she'd make a very convincing mother. She arrived at the part of the script where there's a (very graphic) birth scene, and told herself that she'd never be able to portray it properly, because she'd never given birth. We think that just for a moment, she may have forgotten what 'acting' was.