The singer, who found fame with pop group Liberty X on U.K. talent show Popstars in the early 2000s, gave birth to a baby girl, named Rosie, on Sunday (27Dec15), and she took to Twitter.com on Monday (28Dec15) to share the news with fans.
"The newest addition to our family, beautiful Rosie, arrived safely into the world yesterday," the new mum writes alongside a black-and-white photo of the tot's feet. "We couldn't be happier or more in love xxx".
Pietersen also used social media to celebrate the news, sharing the same snap and captioning it, "Welcome to the world to our beautiful little girl, Rosie!"
The couple, which wed in 2007, also has a five-year-old son, named Dylan.