The blonde singer, 36, has been showing off a noticeably slimmed down figure in recent days, prompting a wave of speculation about whether she was on a new diet or hoping to land a fitness DVD.
Claire set the record straight on ITV's Loose Women today.
"My husband Reece and I decided that we want to try for another baby and me being overweight was causing us a lot of problems," she said.
"I went to see the doctor and he advised that I lose weight. So that is the one and only reason why I have done it this time."
Claire added: "I'm not bringing out a fitness DVD, I'm not doing some miracle diet, I don't have the secret to weight loss and staying slim – I clearly haven't because I've been up and down so many times."
This was the first time Claire had publicly spoken of her desire to get pregnant again and that her weight could have made it more difficult for her.
"I actually realised that whenever I have become pregnant, I was slim. I've never tried [to get pregnant] when I was overweight before and this time it just wasn't happening so we went to find out what was going on and that was the first thing."
Claire also clarified that she is not pregnant yet – losing the weight was to boost her chances of conceiving a sibling for kids Charlie, six, and Daisy, four.
Yesterday, Claire had tweeted her frustration at speculation surrounding her weight loss, telling fans: "I'm finding it very upsetting hearing all the nonsense being written about me the last few days so I have decided to set the record straight."
Got an iPad or iPhone? Reveal Mag is specially designed to fit!