Gwen stepped out in a loose-fitting navy and silver halter neck gown at Beverly Hills' Wallis Annenberg Center for the Performing Arts Inaugural Gala.
The 44-year-old swept her blonde hair into an elegant bun and accessorised her outfit with a metallic clutch bag and bright pink lipstick while her husband Gavin Rossdale wore a smart black dinner suit and bow tie.
The couple, who have been married for 11 years, joined the likes of Demi Moore, Nicole Richie and new dad Josh Duhamel at the gala.
Gwen, who is already mother to sons Kingston, seven, and five-year-old Zuma, sparked rumours that she was pregnant in August when she was seen in London showing a more rounded stomach than usual under a navy blue trench coat.
In September the rumours escalated when the No Doubt singer was spotted taking Zuma to his LA school dressed in loose grey trousers and an oversized blue top.
A source close to Gwen and Gavin also reportedly revealed the news to US magazine In Touch Weekly, saying that the couple are over the moon about the pregnancy.
"Gwen and Gavin couldn't be happier," said the source. "She's just trying to focus on resting right now. She wants to make sure that all is well with the baby."