The stylist shared a photo of the glamorous pair comparing their bumps, with the following caption: "Another fun one from moms night out at #wallisannenberg! Isn't @gwenstefani the most gorgeous? #majormommymoment"
Followers were quick to point out that the snap – taken at the at Beverly Hills' Wallis Annenberg Center for the Performing Arts Inaugural Gala on 17 October – was proof of Gwen's happy news.
One fan, Beaker555, wrote: "either this is absolute confirmation @gwenstefani is pregnant, or she's eating a lot of ice cream just to mess with us..."
Gwen gave nothing away when she arrived at the event in a loose fitting metallic halter kneck gown with husband Gavin Rossdale. But in Rachel's photo, as she cradles her tummy, there can be little doubt that the couple are adding to their family.
The platinum blonde star, 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."