The 25-year-old actress, who strips off for her new film 'Factory Girl', admits her body only appears flawless on screen because of make-up and clever camera work.
She told Britain's Closer magazine 'They spray make-up all over your body so you don't see the stretch marks that we girls all have.' Although she hates her unsightly stretch marks, Sienna is comfortable with her weight and refuses to go on a diet.
The blonde star - who was horrified that her breasts shrank to a 32B cup size after she slimmed down for her role as Edie Sedgwick in the movie - said: 'I don't eat salads and I don't drink skinny coffee. I do run around a lot but I'm not healthy.' Sienna fears that one day her lack of exercise and penchant for partying will catch up with her.
She added: 'One day I'll wake up and my bum will be on the floor!'