Walsh announced last month that she is expecting her first child with boyfriend Justin Scott, reported Contactmusic.
"We were all shocked that Nadine was the first because we thought it'd be Kimberley. I think Kimberley will be the bossy mum, whereas Nadine has that LA thing going on," Harding said.
Harding, 32, has not seen Nadine Coyle, who lives in Los Angeles with her NFL player boyfriend Jason Bell, since she gave birth.
"I haven't met her baby yet, but she's getting on very well, surprisingly! She's really laidback, and so is the baby - she sleeps through the night.