The 40-year-old beauty found out she and husband Justin Scott were expecting another baby in September 2020, just a month after her former Girls Aloud bandmate - who died in September 2021 aged 39 - revealed she was battling advanced breast cancer.
Though she knew it was "weird" to hide the happy news from her friend, she felt it for the best at the time because Sarah knew her chemotherapy treatment would stop her starting a family of her own naturally.
Speaking to Sophie Ellis-Bextor's 'Spinning Plates' podcast, Kimberley grew emotional as 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.
"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.'
"And I just worked it up and I think I just thought 'No, I can't tell her at this moment.' "
But Sarah found out eventually and while Kimberley - who is now mother to sons Bobby, six, Cole, four and 15-month-old Nate - was "embarrassed" when her friend raised the issue, she was relieved for it all to be out in the open.
She laughingly continued: "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.
"But she was just amazing. She was like, 'My mum's just seen a picture of you, I can't believe you're pregnant again.'
"So I FaceTimed her. We had a long conversation and I felt so much better."
Kimberley praised the strength Sarah showed throughout her illness, in particular when it came to speaking about her awareness she would never get pregnant.
She said: "'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.
"She was amazing and she did speak to me about that - about the fact that she wasn't going to have kids and we were really honest about it.
"'When she finally came down and met the baby and spent some time with the boys she was actually loving it, and asking all these questions. It completely broke me. But I'm so, so grateful that I had that time and the kids had that time."
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