The Forgetting Sarah Marshall star became engaged to the actor in 2009 and they are now set to become first-time parents.
A representative for Bell has confirmed the happy news to Usmagazine.com.
The actress previously revealed she doesn't want to wed until same-sex unions are made legal across the U.S., telling The Advocate magazine, ""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 best 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. Period.""