Kristen Stewart felt like "an absolute nutcase" during the birth scene in The Twilight Saga: Breaking Dawn Part 1.

The American actress reprises her role as Bella Swan in the upcoming movie, which is the first instalment of the two-part finale of the popular vampire film series.

The film sees Bella and vampire Edward Cullen - played by Robert Pattinson - marry and have sex for the first time. Bella falls pregnant after their night of passion, and gives birth to a half-vampire baby.

The PG-13 rating limited filming with regard to the intense birth scene.

"That's what I'm really disappointed about and I've talked about it with Bill Condon, director" Kristen told Entertainment Weekly. "It was more sympathetic or something when I played the scene with less energy, and it made it easier for him to tell the story. But in the book, Bella is screaming, NOOOOO!'. And I did it like that crazy and Exorcist-like.

"I was going for this weird mix of turning into the most feral mother you can imagine and also fighting for my own life. I mean, I was a nutcase. I was an absolute nutcase."

Taylor Lautner who plays werewolf Jacob Black in the series agrees with the sentiment. However, the actor is still impressed with how the scene turned out.

"I do still love the birth scene," he added.

"I know it's frustrating that some things aren't in there."

Robert and Kristen's sex scene was also edited with the film's young audience in mind.

The pair were disappointed that the passion was cut out at times.

"There's a version where it was really intense," Robert revealed. "Cinematically, cut all together, it's awesome. But we didn't really do that scene," Kristen added, before Robert chimed: "And when we did, they kept telling us to stop."