Kate Middleton is back in great shape just a few months after delivering her third baby, Prince Louis.

Earlier this week, many were impressed upon seeing the Duchess of Cambridge in skinny jeans while playing tennis at Basildon Sporting Village. Her slim figure was undeniable and she looked impeccable with her curls and makeup.

According to Zoe Forsey, a journalist for Mirror, Prince William's wife might still be using the same technique she did before welcoming Prince Louis. Middleton reportedly followed a fitness program written by Kelly Rennie. In fact, the duchess wrote Rennie a note to thank her for the help it offered.

Rennie lives in Christchurch, New Zealand and has transformed her own body by living by her own diet plan. Here's a glimpse of Rennie's program.

Daily diet at her heaviest:

Breakfast: Cereal or toast

Lunch: Sandwich, crisps and a chocolate bar

Dinner: Fast food takeaway such as McDonald's with an unhealthy dessert

Daily diet now:

Breakfast: A "power shake" made from almond milk, natural protein, berries, avocados and half a banana

Lunch: Salad with chicken, peppers, broccoli, carrots

Dinner: Chicken stir-fry with rice and lots of vegetables

Aside from following Rennie's program, according to Isabella Sullivan, a journalist or Daily Express, Middleton is popular for practicing Dukan diet. This includes high-protein, low-carb French diet that a number of celebrities including Jennifer Lopez and Nicole Kidman follow.

However, this type of diet is not for everyone as it involves working in for intense stages - attack, cruise, consolidation and stabilization. In the first stage, one is only allowed to eat protein not even fruit or vegetables.

In the attack stage, one will slowly introduce vegetables by consuming it on some days. In the consolidation stage, fruits, carbs, cheese and two celebratory meals are allowed. For the stabilization stage, one is required to stick to a protein-only diet once a week.

Middleton is very strict with her diet. Following her pregnancy, she ate diet-friendly foods like avocados, berries and porridge. Now, she maintains her weight by eating ceviche, gazpacho, watermelon salads and tabbouleh.

Aside from being picky with her food, Middleton revealed that her active lifestyle has helped her shed the extra weight she gained during her pregnancy. "I am running after my kids," Middleton told Professor Bulbul Dhar-James about her successful weight loss when they visited India in 2016.