"Harry and Megan were not about to post anything the way they did two years ago," added the author, referencing the fact that in years prior, the Sussexes marked her special day with a tribute on social media.
According to Anderson, the mom-of-three had a "very low-key" party with "a very small circle of friends." She also partook in her tradition of baking with her mother, Carole Middleton.
In honor of her birthday, the royals released three beautiful photos.
"The three new portraits which have been released to mark the occasion will be shown in the community in three meaningful places: Berkshire, St Andrews, and Anglesey, as part of the @NationalPortraitGallery nationwide ‘Coming Home’ exhibition, ahead of the Gallery reopening in 2023," the caption read.
In one of the photos, Kate wore a pair of earrings that belonged to Queen Elizabeth, and in another, she donned jewels that Princess Diana once wore.
Photographer Paolo Roversi revealed that the royal was actually a bit "apprehensive" about the photoshoot, but all went smoothly.
"Every day she is machine-gunned by photographers but not used to posing; knowing my photos with the models she was a bit fearful in facing a real session, which then required about four hours of work," he explained. "But once she started it would be very easy, I reassured her. And so it was."
Expert Ingrid Seward raved over the pictures, noting that her confidence hints at a successful future.
"Kate is coming into her own. She is genuinely involved in her charity work, which will be her legacy," she continued. "We can see Kate stepping up a level, the coming of a queen in waiting, and I think she is more [than] ready to fill those shoes."