The 'What About Us' singer looked beautiful and happy in her sparkly costume as she rocked the Manchester O2 Apollo with her bandmates on Sunday, 14 September.
Una, 32, announced her second pregnancy last week, just a day after The Saturdays kicked off their nationwide 'Greatest Hits Tour'.
She and rugby star husband Ben Foden, 29, are already parents to daughter Aoife Belle, two.
Revealing their happy baby news via Instagram, Una said: "I know lots of people have been guessing and Ben and I can now confirm that Aoife is going to have a baby brother or sister next year!
"We're all very happy!"
After being inundated with congratulatory messages, the red-headed singer tweeted: "Thank you so much for all the well wishes received it means a lot."
Of course, her Saturdays bandmates were among the first to offer their congratulations, with Mollie King, Vanessa White, Frankie Bridge (née Sandford) and Rochelle Humes all gushing about their excitement.
This will be the fourth Saturdays baby.
Frankie and footballer husband Wayne Bridge are parents to son Parker Bridge, who turns one next month, and Rochelle and hubby Marvin Humes are parents to daughter Alaia-Mai, one.
Celebrities who welcomed babies in 2014!
Copyright: Rex Features WENN/Matthew Taylor/Daniel Deme/Josiah True/Matthre Taylor/Joe
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