Walsh became pregnant for the third time in September 2020 just a month after Harding made public that she had been diagnosed with advanced breast cancer and as a result of chemotherapy would not be able to have children.
Appearing on Sophie Ellis-Bextor's podcast Spinning Plates, Walsh discussed her guilt over being pregnant while Harding was terminally ill.
She said: "It was the most horrific time in some ways for me personally, because shortly after I think I found out she was poorly I got pregnant again and I almost didn't want to tell her."
The Girls Aloud singer added: "I'm getting really emotional thinking about this. In reality I kind of knew she's not going to have any kids == she isn't going to be able to — and she always asked me about the boys all the time.
"And I didn't tell her for a while. And I knew it was weird because I was like 'I should call her and tell her, like it's amazing news'."
Walsh also revealed that Harding ended up finding out anyway: "Sarah being Sarah just called me up and she was like 'Ay, I've just seen you're having another one, are you alright?' And I was like 'Yeah, I know I'm sorry', and I was so embarrassed for not calling her but I couldn't bear to give her another moment of pain."
She also said of her late bandmate: "She was amazingly strong. It just blew me away because honestly in the band I don't know if I really saw the strength in the same way I saw all of her vulnerability and all the other sides of her. And I don't know if I ever gave her credit for the strength that she did have, because when it actually came to it, it was mind-blowing."
After being diagnosed with breast cancer, Sarah Harding later died of the disease in September 2021. Since Harding's death, Girls Aloud have reunited several times to raise money for cancer research.
Watch below: Kimberley Walsh says Girls Aloud may never perform again without Sarah Harding