But rather than get bogged down by the inevitable parallels, the Duchess of Cambridge would be wise to take a pointer or two from the late royal, particularly when it comes to rearing her firstborn in the ever-scrutinizing global spotlight.
Lesson one: Keep the third in the line of succession as grounded as possible. Which, granted, may be easier said than done.
"Diana was always very keen on William and Harry knowing the world around them," Victoria Arbiter, ABC's royals contributor and daughter of former Queen Elizabeth II press secretary Dickie Arbiter, told E! News."She took them to McDonald's, she took them to the movies, they went to theme parks and homeless shelters.
"Yes, they were born into a very privileged background, but Diana made sure they knew they lived in a privileged background that wasn't normal and knew how other people lived."
It's a lesson that Will and Kate have already seemed to take on board, even before they revealed their happy royal baby news.
"That is how William and Kate are; they are very down to earth," Arbiter said. And Kate has already shown similarities with Diana, and not just (though yes, that as well) with her fashion sensibilities.
"Really, we are going to see quite similar mothering from Kate. She is going to be like Diana. She is going to want to be present; she is going to want to take the children to school in the morning. Diana made sure she was on hand for everything she possibly could."
And while the late princess would no doubt have been the same as a grandmother, one of the best friends of the late royal shared with E! News just how ecstatic Will and Harry's mother would have been to share in the pregnancy news.
"This is the greatest gift, to have a new heir to the throne," Diana's close pal, the handbag designer Lana Marks, told E! News.
"Diana would be overjoyed. It's spectacular news. She will be smiling from ear to ear from above."
Especially since—like any proud parent, noble or not—she was certain, even when Wills was a young boy, just how good a dad he would become.
"Diana always spoke about William's even temperament and what an incredible father he would make," Marks said. "She always talked about his sense of duty and his empathetic nature. She was grooming him for this. She knew one day he would be king and with that comes the responsibility of producing an heir. That is the greatest gift he can ever give the queen.
"He is a king of the people. It's pure magic."
—Additional reporting by Melanie Bromley