Melanie Hamrick, the American ballet dancer, 29, has been dating the Rolling Stones frontman for the past two years, and now sources tell U.K. newspaper The Sun she is about three months pregnant with their first child together.
The insider says the couple is "surprised and happy" for the impending arrival, adding Mick isn't "fazed" by the baby news, even though the little one will be his eighth child.
"He's been incredibly supportive," the source tells The Sun. "She's very bright, intelligent, sparky and has her own ideas about how she wants to lead her life. She knows what a great relationship Mick has with all of his children and can't wait for him to be a dad again. They are taking the news in their stride."
Mick and Melanie first sparked romance rumours in June, 2014, just three months after his fashion designer girlfriend L'Wren Scott committed suicide.
At the time, the rocker was photographed on a hotel balcony in Switzerland with the ballerina and they have been seeing each other regularly ever since. While the baby will be Melanie's first, Mick has seven children with four other women, including four kids with his former longtime partner, supermodel Jerry Hall.
Mick's surprising baby news comes just two months after his bandmate, 68-year-old Ronnie Wood, welcomed twin girls with his theatre producer wife Sally Humphreys.
Wood has four adult children from his previous two marriages to the late Krissy Findlay and Jo Wood. Sally, who is 30 years his junior, wed Ronnie in 2012 after a whirlwind six-month romance.