The 32-year-old mama-to-be looked every bit the fashionista in a Mary Katrantzou floral dress which perfectly hid her bun in the oven.
The Hart of Dixie star paired the look with Bionda Castana white pumps with black ankle straps.
Bilson chatted with E!'s own Kristin Dos Santos during the event and gushed about working on the upcoming season of her hit show.
"I guess I felt pretty good because we have a solid thing going at this point and we have so many fun things to play with and it's so different from any other show on the network," she explained.
"I'm lucky with all my co-stars and also Zoe is an idiot and it's fun playing an idiot."
Bilson also joked that she felt like she was one of the older stars on the CW roster this season, saying, "I'm like an old timer here with all these youngins' and their new shows and I'm old mammy." Hardly!
Earlier today E! News confirmed that the adorable brunette is expecting her first child with longtime beau Hayden Christensen.
The good-looking duo began dating in 2007 after meeting on the set of the action-packed film Jumper.
The co-stars got engaged in December 2008 but called it off in August 2010, only to reconcile three months later.