Fiona Cairns revealed to the Daily Mail's Weekend magazine that the couple had saved the top three levels for themselves, instead of the usual one.
Newly married couples usually save the top tier of the cake for the christening of their firstborn, but the Duke and Duchess's decision to save the top three levels has prompted speculation that they may be planning to have more than two children.
The pastry chef has also said that the top level would certainly keep long enough to be eaten at Prince George's christening, which is set to take place later this year.
"I'm not even sure how they've stored the cakes. If they wanted to use one for Prince George's christening it would be fine, though," she said.
"The icing would have to be redone but the cake itself will be as good as it was on the day."
The cake, chosen by Kate herself, featured 17 different flowers, was more than one metre tall, weighed 100kg and took a team of six people three days to complete.
Mrs Cairns, who also created HELLO! Magazine's 25th birthday cake, said making it was "the most stressful thing I've ever done", and admitted to breaking down in tears when it was being set up in Buckingham Palace.
She added that Kate had been "one of the most clued-up brides I've had", and had given her clear instructions for the design.
"She had very definite ideas about what she wanted," Mrs Cairns said. "The cake was to tell a story, along with the dress."
But all the hard work seemed to pay off in the end as the newlyweds told pastry chef Fiona that the creation had surpassed their expectations. "At the end of the day, all that matters with wedding cakes is that the couple involved are happy with it."