The 44-year-old singer looked stunning in the sparkling Salvatore Ferragamo dress as she walked down the red carpet with rocker husband, Gavin Rossdale.
The new baby will be joining the couple's two older children Kingston, 7, and Zuma, 5, as well as half-sister Daisy Lowe, 24, from Rossdale's relationship with Pearl Lowe.
Back in September of last year, Stefani said she and Rossdale, 47, were looking forward to having another baby.
"I really, really, really wanted one about two years ago. And it didn't really work out," she said. "So, I feel good with what we've got. Everything works out how it should. You can't plan anything, right? You can try."