The pop beauty and her England rugby star boyfriend Ben Foden - who she has been dating for three years - are expecting their first child together and are "really excited" about the baby, which she is due to give birth to in March.
Una said: "I'm really happy! It was totally unexpected, but me and Ben are really excited."
They said in a statement: "We're so excited for Una! We can't wait to have a sixth Saturday. Ben and Una are going to be fab parents."
Una returned to her hometown of Tipperary, in Irleland, earlier this week to break the news to her parents and was "overwhelmed" by their reaction.
A source told the Daily Mirror newspaper: "All the girls are absolutely overjoyed for Una - they can't wait to meet the first Saturday baby!
"He or she is going to have four very excited aunties when they arrive in late March.
"All of Una's family are really supportive, too. She was overwhelmed by the response when she went home.
"Ben's delighted. He's looking forward to celebrating fully with Una when the Rugby World Cup is over."