Prince Harry was born on September 15, 1984, a second son for Prince Charles and Princess Diana after the birth of Prince William in June 1982. The Duke of Sussex, who turned 41 on Monday, reflected on his birthday in his memoir, Spare, recounting how his father declared to Diana, “Wonderful! Now you’ve given me an Heir and a Spare—my work is done.” But while speaking to biographer Andrew Morton, the late princess shared her memories of the day Harry was born, revealing a surprising account of the summer leading up to his arrival.

“Between William and Harry being born, it was total darkness,” Diana admitted to Morton, who authored Diana: Her True Story—In Her Own Words. “I can’t remember much; I’ve blotted it out. It was so much pain. However, Harry appeared by miracle.”

Describing the summer of 1984, the late royal said her difficult relationship with King Charles had been better than ever. “We were very, very close to each other the six weeks before Harry was born, the closest we’ve ever, ever been and will be,” she admitted.

However, after her second son was born, the couple grew distant. “Then suddenly, as Harry was born, it just went bang, our marriage, the whole thing went down the drain,” Diana told the biographer. She also shared that she “knew Harry was going to be a boy” after seeing him on an ultrasound.

“I knew Harry was a boy and I didn’t tell him,” Diana admitted to Morton. Charles, who’d hoped for a daughter, didn’t react well to the news. “First comment was: ‘Oh God, it’s a boy,'” Diana shared. “Second comment: ‘And he’s even got red hair.”

In the book, Diana continued that “something inside” of her “closed off” after the comment, but that ironically, Prince Harry was “actually closer to his father than perhaps William” at the time she spoke to Morton in 1991.