According to Andrew Morton's book on Diana's life, Diana: Her True Story, Di reportedly revealed the heartbreaking sentence Charles said to her when Harry was born.
The son of Queen Elizabeth wanted their second child to be a girl, so when Harry was born, he said: "Oh God, it's a boy," express.co.uk reports.
Although Charles said he was 'joking', Diana was very hurt by the comment.
The birth of Harry is said to have marked the breakdown of their marriage, with Charles making more hurtful comments about the child, causing Di to shut down from her husband.
Charles spoke about his son's hair colour, saying "he even got red hair", in reference to the trait from Diana's side of the family.
Although Di desibed Harry as a "miracle", when he entered the world her relationship with Charles went "down the drain".
The couple announced their divorce in 1996. Diana died one year later.