The Princess of Wales revealed in a 1991 interview that right before giving birth to her second child, Prince Harry, she and Charles had become "very, very close to each other," but that all changed when five words were spoken.
According to the book Diana: Her True Story by author Andrew Morton, the late princess revealed the heart-breaking sentence to be, "Oh God, it's a boy", which was said once Harry had been born. Although Charles insisted he was just "joking" the damage had already been done and Diana was hurt by his comments.
Prior to giving birth, Diana knew what the gender of the child was before Charles knew, "beacuse she saw the scan but kept it a secret because she knew he 'wanted a girl'."
Charles went on to further complain to Diana's mother, Frances Shand Kydd, over the gender and said he was, "so disappointed - we thought it would be a girl."
Diana recalled that Ms Kydd was so furious at his comment, she "snapped his head off" and told him he should "realise how lucky you are to have a child that's normal."
It wasn't the only time Charles made hurtful comments towards Diana about their newborn son. After Harry's birth, Charles spoke about how his son had "even got red hair", which is a trait that runs in Diana's family.
Diana stated that Harry was "a complete joy" and a "miracle" for her life, but that the comments made by Charles marked the beginning of the end of their marriage as their relationship went "down the drain".
Princess Diana and Prince Charles wed on 29 July 1981 at St Paul's Cathedral in London, in a ceremony viewed by millions globally.
However, Diana claimed that "something inside me closed off" once she heard Charles' comments and they divorced in 1996, less than a decade after Harry's birth.