After Kate was admitted to hospital with a rare form of severe morning sickness the Duke and Duchess were forced to go public with the baby news much earlier than intended.
The 30-year-old Duchess, who found out last month that she was expecting, has not yet reached the 12-week stage of her pregnancy, when couples usually share their news.
She was taken ill while staying with her parents in Bucklebury, Berks, and drove with her husband to the King Edward VII Hospital in London, where she is expected to remain for several days, the Telegraph reported.
The Duke of Cambridge spent Monday at her bedside. He was seen leaving the hospital at 8.20pm and is expected to be granted compassionate leave from his RAF Search and Rescue duties.
The Duchess's illness prompted a scramble to tell members of the Royal family about the pregnancy before news of her arrival at hospital leaked out.
Royal aides said the decision to go public "was very much driven by the Duke and Duchess", who were aware that it would be impossible to keep the news a secret in the age of Twitter.
"It's a very hard thing to go public at such an early stage, but they wanted to be open with people as much as possible," an aide said.
The Queen, the Prince of Wales and Prince Harry, who was contacted in Afghanistan, were all told the news, before St James's Palace made the announcement at 4.01pm.
"Their Royal Highnesses the Duke and Duchess of Cambridge are very pleased to announce that the Duchess of Cambridge is expecting a baby. The Queen, the Duke of Edinburgh, the Prince of Wales, the Duchess of Cornwall and Prince Harry and members of both families are delighted with the news," a spokesman for St James's Palace said.
"The Duchess was admitted this afternoon to King Edward VII Hospital in Central London with hyperemesis gravidarum. As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter," the spokesman said.
The announcement was followed 10 minutes later by a tweet from Clarence House, the first time a royal pregnancy has been confirmed on Twitter.
The Duchess is reportedly being treated by Marcus Setchell, the former gynaecologist to the Queen, who delivered the Countess of Wessex's two children as well as Leo Blair, the son of the former prime minister.