"It's very new and they just found out," the source tells Us. "They're excited!"
Married since 2002, Stefani and Bush rocker, 47, are already parents to sons Kingston, 7, and Zuma, 5. Rossdale also learned in 2004 that he is dad to daughter Daisy Lowe, 24, from his brief relationship with singer-turned-designer Pearl Lowe.
During a vacation with her family in the South of France on Aug. 16, Stefani showed a slight tummy while wearing a pink bikini top and striped, side-tie bottoms. The "Wind It Up" singer took it easy while relaxing on a boat and soaking up the sun.
In an October 2012 interview with Marie Claire, Stefani revealed that she had wanted to have a third child. "I really, really, really wanted one about two years ago. And it didn't really work out," she said, without going into specifics. "So. . . I feel good with what we've got. Everything works out how it should. You can't plan anything, right? You can try."
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