The marriage between Prince Charles and Diana, Princess of Wales, was never easy, but this became the point of no return.

Daily Mail/REX/Shutterstock

“I hope it will be a girl this time,” Prince Charles said to a well-wisher in London, according to the Daily Mail. The year? 2014. The occasion? Kate Middleton’s pregnancy. But for some close to the late Princess Diana, the comment brought back bitter memories of the moment the beloved “People’s Princess” knew her already-troubled marriage to Prince Charles was doomed.

In the 21 years since Princess Diana’s tragic death, many secrets about her marriage to Prince Charles have surfaced. One of those secrets had been revealed by Diana to her biographer, Andrew Morton, who wrote of it in his 1992 book, Diana: Her True Story, but it appears to have been largely glossed over by the public until the birth of Princess Charlotte in 2015. And it’s come to light again recently as Prince Harry and Meghan Markle start their family. The revelation of the remark is only part of what has emerged about what really happened between Prince Charles and Princess Diana.

That secret is that when Diana was pregnant with Prince Harry, Prince Charles had been desperately hoping for a baby girl, and upon Prince Harry’s birth, Prince Charles’ very first comment cut Princess Diana to the core: “Oh God, it’s a boy,” he reportedly said.

Although Charles dismissed his comment as mere “joking,” new mummy Diana felt deeply hurt for a number of reasons. First, in the weeks leading up to Prince Harry’s birth, Diana and Charles had been working at their marriage, and had grown “very, very close,” The Mirror reports. Knowing that Charles had been hoping for a daughter when Diana learned she was carrying a baby boy, she kept it from Charles so as not to spoil their bonding and hoped that the birth of a healthy son would delight Charles and make him forget his preference. That is, of course, not what happened.

Add to that Charles’ second comment: “And he’s even got red hair.” Another “joke” that went right to Diana’s heart. For one thing, Diana’s family tree was chock full of redheads, so the comment was insulting on a primal level. Additionally, Diana may have felt that Charles was needling Diana about the extramarital affair she had had with red-headed James Hewitt (the affair had ended two years earlier, so Hewitt couldn’t have been the father).

Finally, Diana had just endured nine hours of natural birth, which followed a miscarriage before she became pregnant with Harry.

Although Charles knew that Diana was sensitive about the topic, at Prince Harry’s christening, he brought it up again, this time to Diana’s own mother. “We were so disappointed—we thought it would be a girl,” he reportedly said to Frances Kydd, who bristled at the comment and reported it to Diana.

And with that, “something inside me closed off,” Diana told Morton. As “suddenly as Harry was born it just went bang, our marriage, the whole thing went down the drain.” The couple didn’t officially divorce until 1996, 12 years later. Read on to find out how Diana really spent the final weeks of her life.