Kate shared a heartfelt message of thanks on Sunday evening in the UK, writing: "Thank you for all of your very kind birthday wishes, and to Paolo and the National Portrait Gallery for these three special portraits. C"
It's safe to say the portraits sent royal fans into a frenzy, but in all the excitement there were plenty of details the average viewer missed.
From what was playing as the portraits were snapped, to the hidden message in her jewellery – here are the secret details in Catherine's new birthday portraits.
A touch of rock ‘n roll
While most royal portraits are elegant, there's something about Kate's latest series that sets them apart from the pack.
The future queen consort appears more relaxed and intimate than ever before, and photographer Paolo Roversi revealed the secret behind his shots.
"At first the duchess was apprehensive," he told Italian newspaper Corriere della Sera.
After all, Kate is photographed constantly by the press, the public and royal photographers, and in her personal life she often prefers to be behind the camera.
But Roversi had a clever little trick up his sleeve.
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"In the end I wanted to take pictures in motion, so with that wonderful wide skirt I made her dance in front of my lens, a kind of accelerated waltz mixed with a pinch of rock 'n roll."
He used music to get Kate relaxed, then focused on capturing her natural beauty above all else – and we think he succeeded!
"I didn't want her to look too Lady Duchess, too establishment, but as pure, as contemporary as possible. Or even timeless," he added.
The Duchess also wore minimal makeup for the shoot, kept her hair loose and flowing and opted for simple, yet sentimental jewellery.
A tribute to Diana
The duchess often makes secret statements through her jewellery and chose to do so again for her birthday portraits, incorporating a sweet homage to the late Princess of Wales.
In the first image that was released, Kate sits in profile with her hand resting on her lap, showing off Diana's iconic sapphire engagement ring.
But that wasn't the only nod to the late princess. Gazing off-camera with her hair delicately blowing in the breeze, Kate was also wearing a set of pearl drop earrings.
The elegant pair, known as the Collingwood pearl earrings, previously belonged to Diana and she wore them countless times throughout her years as a royal.
A nod to the Queen
Not one to do anything by halves, Kate took the opportunity to honour another iconic royal woman with her jewellery choices in her second portrait.
Dressed in a dramatic one-sleeved red frock for the photo, the Duchess of Cambridge chose a set of glittering diamond earrings.
This pair was on loan to her by none other than Queen Elizabeth II, who has been known to lend Kate jewellery on several occasions since her 2011 wedding.
In fact, the tradition began when Kate first borrowed the now-iconic Cartier Halo tiara from the Queen's collection to wear on her wedding day more than 10 years ago.
A wedding day throwback
Speaking of her wedding day, the duchess managed to sneak a sweet reference to her nuptials into her wedding portraits too.
Kate chose to wear three dresses from designer Alexander McQueen for the photos; the same fashion house behind her jaw-dropping lace wedding gown.
All three of the dresses – from the off-the-shoulder chiffon gown, to the billowing scarlet number and delicate ruffled frock – have been credited to the British designer.
The duchess has had a close connection to the label throughout her tenure as a senior royal, often choosing McQueen designs for her most important outings.
While she's had plenty of iconic moments in McQueen gowns over the years, there's no denying Kate's wedding dress stands alone as her most beloved moment in McQueen.
This article originally appeared on our sister site, Now to Love.