She said: "I had an inkling I was having a girl. I come from a family of girls. I went to an all-girls school and I'm in a girl band, so I guess it's fitting that I'm having a daughter.
"I felt her kick for the first time last week, and I think she's going to be lively. I didn't mind whether we had a girl or a boy, as long as our baby was healthy."
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Una is particularly excited her band mates Rochelle Wiseman, Vanessa White, Frankie Sandford and Mollie King – who are all currently rehearsing for their first arena tour – will get to be aunties.
She added to Britain's HELLO! magazine: "We tell each other everything. They were so excited they were screaming, but I said, 'Please, this is so early, it has to stay secret for now.'
"And they were so good, they didn't breathe a word. Our baby is very lucky to have those four as doting aunties."
However, one problem she is facing is naming their baby, and Una has so many suggestions she is confusing herself with possibilities.
She said: "It's too overwhelming. I find myself suggesting names I don't even like, knowing I won't get a positive reaction from Ben. I told Ben he should maybe choose the name. My dad chose my name, so why not? We won't go for anything that's in the top ten of baby names. We want to be a bit original but she won't be Pear of Shrimp or anything too off the wall."