Prince William's devotion to Kate Middleton deeper than ever as heir celebrates 'bittersweet' birthday: expert

Prince William is having a "bittersweet" birthday as the heir to the British throne remains a "rock" for his family during a health crisis.

The Prince of Wales, who turns 42 on Friday, has kept up appearances as he continues his royal duties. Both his father, King Charles III, and his wife, Kate Middleton, are battling cancer.

The Princess of Wales put on a brave face when she attended Trooping the Colour on Saturday. It marked her first appearance at a public event since her diagnosis was made public earlier this year.

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"William could not have been a more devoted husband than he has been to Kate," Christopher Andersen, author of "The King," told Fox News Digital. "That’s ever more evident in the way that he has stepped up to reassure the children and, at the same time, take up some of his father’s duties while Kate is undergoing chemotherapy."

"Kate’s public statements about her cancer battle have been nothing less than stunning in their graciousness and insight," Andersen shared. "William recognizes that."

Andersen says William has been more devoted to his ailing wife than ever as he looks after their three young children.

"William also realizes that he is the only one who has to reassure the children that this is just a rough patch and their mother will get through it — not to mention their grandfather, who the children know is also suffering from cancer," Andersen explained.

"George, Charlotte and Louis are also very close to their grandfather the king, and people often forget that," Andersen continued. "It’s up to William to make sure that not only are they reassured about their mother’s situation, but also their beloved grandfather’s. So much is on William’s shoulders as he turns 42. It really is rather staggering."

In March, the Princess of Wales announced she was undergoing chemotherapy for an unspecified form of cancer.

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"I am making good progress, but as anyone going through chemotherapy will know, there are good days and bad days," the princess, 42, said in a statement released ahead of the military parade. She added that she faces "a few more months" of treatment.

Kate said she is "not out of the woods yet" and officials stressed that the engagement didn't herald a full return to public life.

Charles, 75, disclosed his cancer in February and has recently eased back into public duties.

"[This] has to be a bittersweet moment for William, who for the first time has to be thinking about issues of mortality relating to his wife and father that, of course, he has never thought of before," said Andersen.

Still, the ongoing health woes won’t stop the couple from celebrating privately with their children.

"The way that William and Kate most enjoy spending milestones like birthdays and anniversaries is with the children," he said. "And that’s exactly what they will do… The way they celebrate privately is the way the rest of us do, with party favors, gifts and balloons — the whole nine yards. Don’t forget, Kate’s mother… founded a children’s party supply company that was immensely successful. They know how to celebrate birthdays!"

Royal expert Ian Pelham Turner agreed that William, who has been described as a hands-on family man, will prefer to celebrate "behind closed doors" with his wife and children.

"The children will rally around, perhaps creating breakfast in bed, rolling up their sleeves in the kitchen and baking for their father," Pelham Turner told Fox News Digital. "Kate has a passion for baking birthday cakes… She has a tradition of staying up to midnight utilizing huge amounts of cake mix and icing, producing over-the-top cakes."

"Prince William is a family man at heart and loves nothing more than low-key family time connecting with his wife, his rock… and his three children," added British photographer and broadcaster Helena Chard. 

"[William] will want nothing more than quiet, private family time. Let’s hope Princess Catherine also has an up day and enjoys baking a special homemade, elaborately decorated birthday cake with their children — a super way to celebrate and round off Prince William’s busy schedule."

Several royal experts agreed that despite William keeping busy fulfilling royal duties and supporting his father, the Princess of Wales and her well-being remain his top priority.

"While he’s had to undertake many engagements recently alone, he and Princess Catherine are more in sync than ever," British royals expert Hilary Fordwich told Fox News Digital.

"They have been mutually supportive of one another even more so since her ghastly cancer diagnosis, which has enhanced their ever-strong marriage," she explained. "They relish family time together with their three children. Family comes first for them, so they will be spending his birthday all together."

"After Saturday’s Trooping the Colour, it was obvious the looks of love and affection between William and Kate were constant in front of tens of thousands of royal enthusiasts," Pelham Turner shared.

"He is a wonderfully supportive husband to Catherine in a truly difficult time," said royal expert Richard Fitzwilliams. "He is a role model as heir to the throne and… the perfect father… Happy birthday to him. He knows what service means."

The couple first met while studying at St Andrews in Scotland. In the couple’s 2010 engagement interview, Kate admitted that due to her shyness, it took some time for the pair to get to know each other. 

As they spent more time together, they became "very close friends." They moved in together with friends in 2002. Then, in 2003, the pair moved in together and kept their relationship under wraps for about a year. The couple's romance went public in 2004.

In 2007, they split and got back together. William went on to propose in late 2010. And in 2011, they married at London’s Westminster Abbey.

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When Kate spoke out about her cancer diagnosis in March, she noted how William had been a supportive spouse as she attempted to navigate her illness.