The singer is expecting her first child with footballer fiancé Wayne Bridge.
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Announcing the news on Twitter, she wrote: "Some big news from me today! Wayne and I are having a baby. We are both SO happy and can't wait for his or hers arrival!"
Frankie's The Saturdays bandmate Molly King tweeted a photo of the group touching Frankie's bump with the caption "excited aunties":
She said in a recent interview: "I'd love to have a baby with Wayne. He's amazing."
Frankie is the third member of The Saturdays to fall pregnant, as Una Healy is already mum to one-year-old Aoife Belle, and Rochelle Wiseman is due to give birth any day now.
Who's the mumma? Kim KardashianWho's the daddy? Kanye WestBaby ETA: Next yearInteresting fact: Kim's difficulty conceiving the couple's second child was chronicled in 'Keeping Up With The Kardashians', before confirming she was pregnant with a son. (credit:Jason Merritt via Getty Images)
Pregnant Celebs(02 of08)
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Who's the mumma? Coleen RooneyWho's the daddy? Wayne Rooney. Maybe you've heard of him.Baby ETA: JanuaryInteresting fact: This will be the third child for the happy couple. (credit:Shirlaine Forrest via Getty Images)
Pregnant Celebs(03 of08)
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Who's the mumma? Sam FaiersWho's the daddy?Baby ETA: Early 2016Interesting fact: Sam confirmed her baby news on Instagram. (credit:Tim P. Whitby via Getty Images)
Pregnant Celebs(04 of08)
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Who's the mumma? Keira KnightleyWho's the daddy? James RightonBaby ETA: Spring 2015Interesting fact: Keira and James tied the knot in May 2013. This will be their first baby. (credit:Steve Granitz via Getty Images)
Pregnant Celebs(05 of08)
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Who's the mumma? Katherine JenkinsWho's the daddy? Andrew LevitasBaby ETA: Late 2015Interesting fact: The couple shared their baby joy with an adorable snap on Instagram of a pair of pink shoes. (credit:Steve Thorne via Getty Images)
Pregnant Celebs(06 of08)
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Who's the mumma? Olivia ColmanWho's the daddy? Ed SinclairBaby ETA: Summer 2015Interesting fact: The 'Broadchurch' actress has two children already with her writer husband, Ed. (credit:David M. Benett via Getty Images)
Pregnant Celebs(07 of08)
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Who's the mumma? Katie HalilWho's the daddy? Howard DonaldBaby ETA: Early 2016Interesting fact: This will be the first couple for the couple, who married secretly earlier this year. (credit:Bauer-Griffin via Getty Images)
Pregnant Celebs(08 of08)
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Who's the mumma? Kelly ClarksonWho's the daddy? Brendan BlackstockBaby ETA: Early 2016Interesting fact: Kelly confirmed her baby news in a typically Kelly Clarkson-esque fashion, by claiming she was "totally pregnant" during a live show. (credit:Bruce Glikas via Getty Images)