The Duke and Duchess of Sussex told the royal family they were expecting a baby while at Princess Eugenie's wedding.
Prince Harry and his wife Meghan - who have Archie, three, and 19-month-old Lili together - pulled aside his brother Prince William and father King Charles during the festivities for his cousin's marriage to Jack Brooksbank in October 2018 to tell them they were going to have their first child.
Harry recalled speaking to William at St George's Hall to break the news, and was thrilled when his brother reacted as he'd "hoped" and instantly urged him to break the news to his wife Catherine, Princess of Wales.
Writing in his memoir 'Spare', Harry said: “We were in a big room, suits of armour on the walls. Strange room, strange moment.
"[William] smiled and said we must tell Kate.”
Harry recalled Catherine was busy but William insisted they tell her immediately.
He added: “She also gave a big smile and hearty congratulations. They both reacted exactly as I hoped — as I’d wished.”
The 38-year-old prince was "warmed" by his father's reaction when he and Meghan broke the news on the same day.
He wrote: “Just before a drink reception for the bride and groom, we cornered Pa in his study. He was delighted to learn that he was going to be a grandfather for a fourth time; his wide smile warmed me.”
The BetterUp CIO revealed he learned he was going to be a dad when Meghan took a series of pregnancy tests at home while he was asleep, and he woke to find them on a dresser next to the “blue box with my mother’s hair.”
And Harry felt the presence of his late mother Princess Diana in the moment.
He wrote: “Let’s see what Mummy can do with this situation."
After seeing the tests were positive, he added: “I thought: Thank you, Mummy.”
Elsewhere, Harry recalled feeling horrified by the line of questioning he faced when William and Catherine - who have Prince George, nine, Princess Charlotte, seven, and Prince Louis, four, together - welcomed their daughter into the world, because it meant he moved down the line of succession.
Insisting he was "very happy" about Charlotte's birth, he wrote: “A journalist questioned me about it as though I’d received a terminal diagnosis... I thought: First of all, it’s a good thing to be farther from the center of a volcano.
"Second, what kind of monster would think of himself and his place in the line of succession at such a time, rather than welcoming a new life into the world?”