Kate will often step out wearing stunning garments which show off her slender figure.
Speaking to the Mail Online, an expert explained it is likely Kate wears a size six and has the tiny waistline of 24 inches.
The Duchess has shown her love of staying active and has taken part in sporting events while on royal duties.
The royal has stayed fit by playing sports throughout her life and is thought to have an interest in skiing, running and hockey.
As well as this, some have speculated on the diet plan Kate uses to stay in shape.
When it comes to her diet, ABC news claimed the Duchess followed the Dukan plan when shaping up for her wedding.
First published in France, the low carb plan has different stages which require slimmers to eat different foods.
In the first stage, those following the diet plan are encouraged to eat foods high in protein before adding vegetables.
Although fruit and dairy are off the table at the start, these are later reintroduced in stage three of the plan.
After having her children, Woman’s World reported she tucked into foods including avocados, berries and porridge to stay in shape.
It has also been claimed Kate will make cook meals from scratch at home to ensure she can watch what ingredients are included.
The Duchess of York, Sarah Ferguson, 59, has opened up about her weight loss in the past.
After piling on the pounds, she became the face of Weight Watchers and claimed it helped her slim down considerably.
Along with changing her diet, she enlisted the help of a personal trainer in order to get fit.
When trying to lose weight, following the high fat and low carb ketogenic diet plan can burn fat quickly.
Slimmers can tuck into foods including meat, eggs, fish and full fat dairy products.
How did Kate Middleton become a royal?
Kate Middleton, or Catherine, Duchess of Cambridge, was born on 9 January 1982 at the Royal Berkshire Hospital, Reading, England.
She was born to Michael and Carole Middleton, a former flight dispatcher and air hostess. The couple founded a party supplies company, Party Pieces, in 1987 – now estimated to be worth £30 million.
Kate has a younger sister Pippa Matthews, 34, and a younger brother James Middleton, 31.
Kate and her family moved to Amman, Jordan, in 1984, where the future Duchess was educated at an English-speaking nursery school.
After moving back to Berkshire the family sent Kate to private school, St Andrews School. She went on to briefly study at Downe House before moving onto Marlborough College where she boarded.
For university the young Kate attended the University of St Andrews in Fife, Scotland. Here she was awarded an undergraduate MA (2:1 Hons) in history of art.
Relationship with Prince William
Kate met Prince William when they were both studying at St Andrews, and both living in St Salvator’s Hall. Famously, William reportedly took a fancy to Kate after she wore a see-through dress during a fashion show.
In 2003 the couple began dating and lived together during their second year of university.
The couple split in April 2007, however, in October they were back together.
Prince William proposed to Kate Middleton with his late mother Princess Diana’s engagement ring in October 2010. The couple were holidaying in Kenya at the time.
On 29 April 2011 they married at Westminster Abbey with a whopping 26 million watching the event live.
Kate wore an Alexander McQueen dress worth a reported £250,000 on the big day.
Kate assumed the title “Her Royal Highness The Duchess of Cambridge” and also become the Duchess of Cambridge on the day.