Prince Charles made a joke after Prince Harry's birth that broke Princess Diana's heart

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While Princess Diana's close bond with her sons Prince William and Harry was always apparent, there's one thing that many people may not know about Harry's birth.

Prince Charles "desperately" wanted a little girl, so Diana, knowing that she was having a boy, kept Harry's gender a secret throughout her pregnancy.

Diana gave birth to a baby boy, Henry Charles Albert David Mountbatten-Windsor, after nine hours of labor and zero drugs in September 1984. In Andrew Norton's book "Diana: Her True Story," Charles' heartbreaking reaction to Harry's arrival was revealed.

"Charles always wanted a girl," Diana said. "Harry was a boy. His first comment was, 'Oh God, it's a boy.' His second: 'And he's even got red hair.'"

While Charles saw his comments as a joke, Diana was extremely hurt and offended by it -- not only did she just endure nine hours of labor but her older sisters and younger brother all had red hair (it's a trait that runs in the Spencer family).

At Harry's christening in December, Charles reportedly told Diana's mother, "We were so disappointed -- we thought it would be a girl."

Diana also made another heartbreaking confession about the weeks leading up to Harry's birth -- Charles and her were "the closest we've ever, ever been and ever will be."

But after Harry's birth, everything changed: "Then, suddenly, as Harry was born, it just went bang, our marriage. The whole thing went down the drain."

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Diana went on to admit that "something inside me closed off. By then, I knew Charles had gone back to his lady."

If you recall, that "lady" was later revealed as Camilla Parker Bowles.

Charles began an affair around 1978 or 1979 with his now-wife Camilla while still married to Diana. Charles and Diana separated in 1992 and officially got divorced in 1996. However, a recently published book, "Prince Charles: The Passions and Paradoxes of an Improbable Life," divulged that there was trouble in paradise as early as their honeymoon.

As his mother's 20th death anniversary nears, it has been rumored that Prince Harry has proposed to his girlfriend of over a year, Meghan Markle. More on that in the video above.