The royal pregnancy announcement on Monday was so unexpected even the Queen had only been aware she had another grandchild on the way an hour earlier.
The Duchess of Cambridge is believed to be only eight weeks gone but her admission to hospital saw the couple break the news early.
According to the Daily Mail's Royal correspondent Rebecca English, the pair had been hoping to keep the news to themselves until Christmas Day.
The Queen was reportedly unaware Kate was pregnant until an hour before the official announcement
She writes: "But in view of the Duchess’s hospitalisation the couple made the ‘reluctant and difficult’ decision to publicly confirm their news.
"Members of the Royal Family – including the Queen and grandfather-to-be Prince Charles – were informed less than an hour before the statement was put out at 4pm yesterday."
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She is being treated at King Edward VII Hospital in central London and is expected to stay in hospital for several days.
On Tuesday Hackney Labour MP Diane Abbot tweeted that the Duchess is being treated by Marcus Setchell, formerly the Queen's gynaecologist, who she says delivered her own son James and Tony Blair's youngest child Leo.
The royal couple sparked baby rumours last week during a tour in Cambridge, after William accepted a romper suit from a well-wisher, telling her "I'll keep that."
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The handmade baby-gro featured a helicopter and the words "Daddy's little co-pilot" - a reference to the Prince's career as a search and rescue captain.
Samantha Hill told The Mirror: "I made it for when his little one comes along, he's a helicopter pilot so his baby will think his daddy is cool.
"When I gave it to him he said 'I'll keep that' and handed it to his aide."
It seemed like the cat was well and truly out the bag when bookmaker Coral suddenly suspended betting on the royal couple having their first child next year.