But incredibly the Duchess used Kelly's postnatal guide after giving birth to Prince George in 2012 and even sent Kelly and business partner Katie Bulmer-Cooke a letter expressing her thanks.
Now fitness model Kelly has delivered Kate a signed copy of her latest book in the hope it will help her shed the kilos after the Duchess gives birth to her third child, due in April.
‘To get the letter back from Kate showed us the programme could work for any mum,’ Kelly, who lives on the Gold Coast, said. ‘It meant so much at the time.
‘That routine was a postnatal programme, whereas this one is a sustainable fitness routine for the months that follow.
If Kate can do it with the first two, she can do it with the third.
‘She just needs to keep the same principles she applied after her first two kids as she obviously has managed to look after herself well!"
Former finance worker Kelly was driven to turn her life after the birth of her second child when she fell on dark days plagued by panic attacks, anti-depressants and weight gain.
At her heaviest, the mum-of-two tipped the scales at 70kg but she decided to battle her own demons with fitness, taking up at-home exercise.
Within 18 months, Kelly had shed 20kg and whipped herself into good enough shape to become a star in the world of fitness modelling.
Kelly said she boosted her metabolism by replacing her vices of fast food and sweets with a diet of wholesome, nutrient-dense foods like chicken, vegetables and power shakes.
Kelly, who now weighs 50kg and wears a slim dress size 10, said:
‘After I had my second child, I unfortunately became a single mum. That's when I got my passion for helping mums.
‘I think the most important thing is being realistic and sustainable. It's a lifestyle approach they can do every day for the rest of their lives.
‘Most mums are emotional eaters, so part of it is letting go of emotional eating. Having some chocolate or a glass of wine is fine, but not every day and not for the wrong reasons.’