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Kate Middleton was admitted to the hospital for a "planned abdominal surgery" last week, but it wasn’t known to insiders ahead of Kensington Palace's Jan. 17 announcement, royal experts suggested to Fox News Digital. 

"Just days before the announcement I spoke to sources close to the family and they gave no indication that anything was afoot," royal expert Shannon Felton Spence told Fox News Digital following a People magazine report that said her surgery came as a surprise to those close to the royal family. 

"I believe the report," Spence said, adding that the princess is "essential to the health and wellness of the monarchy. "On the human side of it, she is the mother to a future king. She is the partner to a future king. Catherine is the stability, normalcy, and warmth of the family for the next two generations. She is also an invaluable tool for the institution, as she is consistently ranked as one of the most popular members." 

Christopher Andersen, author of "The King: The Life of Charles III," suggested that the main reason the Princess of Wales might have kept her impending surgery quiet was for the sake of her three young children. 

HOW KING CHARLES, KATE MIDDLETON'S HEALTH WOES EXPOSE HOLLYWOOD INFLUENCE ON THE MONARCHY: EXPERT 

Kate Middleton looking on

Kate Middleton was admitted to the hospital last week for a "planned abdominal surgery."  (Victoria Jones - Pool/Getty Images)

"Kate is a very private person to start with, but she certainly is most concerned with how this will all affect her young children," he told Fox News Digital. "She obviously doesn't want to concern or frighten them. When you're the age of George, Charlotte, or Louis, of course, it's a scary thing to see a parent go into the hospital. That is one of the principal reasons, if not the principal reason, Kate has sought to downplay the surgery." 

But, he said, in reality, keeping her health issue a mystery has fueled rumors. 

"Unfortunately, the suddenness of the announcement and the mystery shrouding the Princess's condition have only served to fan the flames of speculation and gossip," Andersen said. "It probably would have been better for the Palace to simply describe the nature of Kate's surgery and get it over with. Until they do, the royal rumor mill will just keep churning away. Compare and contrast this with the King's prostate procedure. Had Charles entered the hospital for a ‘planned surgery’ without further explanation, you can be sure there would have been an uproar."

KATE MIDDLETON IS SELFLESS DESPITE HOSPITALIZATION, CONCERNED ABOUT OTHER PATIENTS AND KING CHARLES: EXPERT

King Charles III is also expected to be admitted to the hospital for a "corrective" procedure related to an enlarged prostate. 

Royal author and commentator Duncan Larcombe added that Middleton’s husband, Prince William, has been given the OK by Charles to only carry out light duties, so he can take care of his wife and Princes George, 10, Louis, 5, and Princess Charlotte, 8. 

Kate Middleton walking to church with her kids

Kate Middleton was last seen in public heading to Church on Christmas Day in Sandringham.  ( Samir Hussein/WireImage)

Kensington Palace announced on Jan. 17 that Middleton was admitted to the London Clinic for the planned surgery and would likely remain there for two weeks. 

The princess was last seen in public while walking with her family to Church on Christmas Day in Sandringham and there were no signs then that she was in ill health. 

The palace has also been tight-lipped about what kind of surgery she’s having beyond that it’s abdominal. 

"I haven’t been able to get anything out of anyone about her surgery or condition," Spence added. 

Royal expert Ian Pelham Turner told Fox News Digital that the news of Charles' procedure has restarted rumors that the king could abdicate in the same way Denmark’s Queen Margrethe II did earlier this month.

He said with William taking care of Kate and Charles, and Sarah Ferguson having their own health issues — Prince Andrew’s ex underwent breast cancer treatment and was just diagnosed with skin cancer — the royal family is "now depleted of major members … This would have meant a massive increase in duties for Princess Anne and the Duke and Duchess of Edinburgh." 

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King Charles looking serious

King Charles III will be admitted to the hospital for a "corrective" procedure related to his prostate.  (Chris Jackson - WPA Pool /Getty Images)

He said that many royal observers have been concerned about the effect a "medical catastrophe" could have on a "slimmed down monarchy." 

"Thus, with no Andrew or [Prince] Harry and Meghan [Markle], Princess Anne's similar concern that slimming down would not work has come home to roost," he added. "In reality, it points now to the Firm in disarray and perhaps a major rethink in the Windsor household. At a time of U.K. austerity, spending more money on a potential increase in monarchy size may be frowned upon, but in the long term necessary."

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Anderson believes that the princess’ surgery, due to which the palace says she won’t return to royal duties until after Easter, is clearly more of a "genuine health crisis" than a "minor procedure."

"Just because surgery was ‘planned’ doesn't mean it's routine," he added, suggesting: "The two-week hospital stay and the long recovery period leave no doubt that this was a major medical issue of some sort."

Middleton, 42, hasn’t had many serious health issues known to the public in the past, except during her three pregnancies when she suffered from hyperemesis gravidarum, a severe form of morning sickness, and had to be hospitalized. 

Kinsey Schofield, host of the ‘To Di For Daily’ podcast, called Middleton an "integral part of the future of the monarchy." 

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"William and Catherine somehow define this really beautiful and hopeful vision of the future," she told Fox News Digital. "You can't recreate it because it's authentic and natural. It is effortless. That is why they are so important to today's monarchy. They inspire people to look forward instead of backwards."