Kristen Bell has admitted pregnancy left her feeling ''brain dead''.

The 'Forgetting Sarah Marshall' actress is expecting her first child with fiance Dax Shepard, 37, and while the blonde beauty is enjoying being with child, she admits the feeling resembles being under the influence of the pleasure-releasing hormone Dopamine.

She told Entertainment Weekly: ''You kind of go brain dead when you're pregnant. You're kind of in a Dopamine phase. I'm kind of happy all the time, and dumb.

''This is without question the biggest transition I'll make in my life to date.''

Kristen, 32, is still working on her US Showtime TV show 'House of Lies', but confessed it is proving difficult to film her scenes towards the later stages of her pregnancy as they have to ''hide'' her growing stomach.

She joked: ''They are always trying to hide my pregnancy [on the show]. I'm always hiding behind a box, or a purse.

''I love working on that show more than I can possibly explain... We'll stop filming around February 12, around Valentine's Day (14.02.13), so I'll have a little bit of pregnancy time after.''

Kristen and Dax got engaged in 2009 after two years of dating and although the couple have discussed marriage, they are waiting until gay marriage has been made legal in their home state of California as a gesture of respect to their close friends.

Kristen previously said: ''The reason we're not rushing to get married is because I don't feel appropriate taking advantage of a right that's denied to my [gay and lesbian] friends.

''That's why we've been so hesitant. Dax and I have talked about it a lot, and this issue is very important to both of us. We're just standing up for what we believe in.''