Stefani, 43, showed a hit of a baby bump while on vacation in France in August, and now we know it's not a relaxed fitness regimen changing the shape of the No Doubt singer's normally rock-solid abs.
"It's very new and they just found out," a source said. "They're excited!"
The new baby will have two big brothers, Kingston, 7, and Zuma, 5, and a half-sister, Daisy Lowe, 24, from Rossdale's brief relationship with designer Pearl Lowe.
Back in October Stefani said she and Rossdale, 47, had tried unsuccessfully to give their boys a little sibling.
"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."