Kimberley (31) said Nadine (28) had always been the least broody member of the band.
"We were all surprised because it ended up being the least likely person," she said.
"During the tour, Nadine said it wasn't really in her brain."
But Kimberley, who will be in Dublin this weekend to promote her book Whole Lotta History, added: "I'm looking forward to seeing her as a mum."
Kimberley wasn't the only person left slack jawed.
The Derry native herself admits her pregnancy came as a shock.
"A couple of the other girls had wanted babies for a long time but I said no way. It was definitely a shock, but I was really, really happy," Nadine said.
And it sounds like it Nadine won't be the only "Mama Aloud".
Kimberley and partner Justin Scott are eager to start a family. "We're both really broody," she said.
"Friends laugh at us and say we're the best babysitters. I don't think it's going to be long now.''
But the Something Kinda Ooh! singer has no immediate plans to take a trip down the aisle.
"My family background growing up wasn't the easiest, my mum and dad split when I was six, and it was quite hard. It shaped who I am."