London (CNN) -- Britain's Prince William and Catherine, Duchess of Cambridge, are expecting their first child after 19 months of marriage, the palace announced Monday.
The royal revelation, which followed months of tabloid speculation, came as Catherine was admitted to a central London hospital with acute morning sickness Monday afternoon. She is expected to be in King Edward VII Hospital for several days, the palace said.
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The palace said her illness is hyperemesis gravidarum, which involves nausea and vomiting more severe than typical morning sickness.
Prince William spent much of Monday with his wife at the hospital, the palace said, but he was seen leaving Monday evening.
Why is the duchess in the hospital?
Duchess of Cambridge pregnant
Catherine, Duchess of Cambridge, left, meets 5-month-old James William Davies as she arrives at the Guildhall during her visit to Cambridge on November 28, 2012. Prince William and his wife, Catherine, are expecting their first child, the palace announced Monday, December 3. Click through this gallery to see the her interacting with the younger generation.
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Catherine, Duchess of Cambridge, has proved to be quite the fashionista since her relationship with Prince William catapulted her into the limelight. Frankly, we can't wait to see how her style evolves as her baby bump grows.
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"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 statement said.
Catherine, 30, is not yet 12 weeks pregnant, Clarence House told CNN, so the palace is not announcing a due date for the child.
William and Catherine's child will be next in line to the British throne after William, regardless of whether it is a boy or a girl, the British Cabinet Office said Monday. Planned changes to the law of succession that end the tradition of a boy jumping over an elder sister are already de facto in effect, the Cabinet Office said.
William and Catherine were married at Westminster Abbey on April 29, 2011, in London's biggest royal wedding in three decades.
The baby would be the first grandchild for the heir to the British throne, Prince Charles, and the late Diana, Princess of Wales.
The news immediately became a top trending topic worldwide on Twitter.
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British Prime Minister David Cameron was quick to tweet his joy: "I'm delighted by the news that the Duke & Duchess of Cambridge are expecting a baby. They will make wonderful parents."
British Labour Party leader Ed Miliband tweeted: "Fantastic news for Kate, William and the country. A royal baby is something the whole nation will celebrate."
Catherine is an art history graduate and the eldest child of Michael and Carole Middleton, self-made millionaires who run a party-supplies company.
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Tabloid speculation about a pregnancy increased in September when Catherine substituted water for wine during a state dinner toast in Singapore.
The buzz reached a boil two weeks ago when Australia's New Idea magazine quoted close friend Jessica Hay saying her former schoolmate, Catherine, would soon announce "wonderful, happy news."
Life & Style magazine in the United States published its own "exclusive" with the same Hay quotes of a December announcement, which she said she had on "the highest authority."
A family history website did the math last summer and predicted Catherine would be with child by November, according to a Time.com report. Genesreunited.co.uk calculated that "the average period between a royal wedding taking place and the couple's child being born is 851 days." Based on that, it predicted a pregnancy in November and a birth on August 27, 2013.
CNN's Matthew Fitzgerald and Jamie Hamilton in London contributed to this report.
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Prince William and Kate kiss on the balcony of Buckingham Palace in London after their wedding at Westminster Abbey on April 29, 2011. Two young bridesmaids, Grace Van Cutsem, left, and Margarita Armstrong-Jones, right, seem to have differing views of being in the public eye.
Catherine speaks with two young girls as she presents shamrocks to members of the 1st Battalion Irish Guards at the St Patrick's Day Parade on March 17, 2012, at Mons Barracks in Aldershot.
Catherine joins in a music class during a visit to The Treehouse in Ipswich, eastern England, on March 19, 2012. Her visited marked the formal opening of The Treehouse, a children's hospice service for the counties of Suffolk and Essex.
A young child holds up a banner as Catherine visits Alder Hey Children's NHS Foundation Trust on February 14, 2012, in Liverpool, England.
A young girl offers Catherine a toy rose as the Duchess visits Alder Hey Children's Hospital on February 14, 2012, in Liverpool, England. Interacting with children during visits to hospital or charities is a part of official duties that Catherine seems to enjoy.
Catherine meets children at Nauti Primary School during the couple's Diamond Jubilee tour of the Far East on September 18, 2012, in Funafuti, Tuvalu. Since her marriage with Prince William, the Duchess has reached out to children and schools all over the world.
Eloise Taylor, Catherine and Margarita Armstrong-Jones stand on the balcony of Buckingham Palace after the Colour Parade on June 11, 2011, in London. The parade is a celebration of the sovereign's official birthday, although the queen's actual birthday is on April 21.
Catherine greets a young girl during a walkabout as she visits Alder Hey Children's Hospital on February 14, 2012, in Liverpool, England.
Catherine talks with flag-waving schoolchildren as she arrives for a visit to The Art Room's classroom at Rose Hill Primary School on February 21, 2012, in Oxford, England. Since the beginning of the year, Catherine has become patron of four charities, including the Art Room.
Catherine receives a bouquet of flowers from a young girl during her visit to the UNICEF Centre on November 2, 2011, in Copenhagen, Denmark.
Catherine laughs as she receives flowers from 2-year-old Lola Mackay, who refused to let go of them, during a visit to the CRI Stockton Recovery Service on October 10, 2012, in Stockton-on-Tees, England.
Catherine, right, speaks with schoolchildren during an official visit to the Art Room facilities at Rose Hill Primary School in Oxford on February 21, 2012.
Catherine talks with a young girl as she arrives for a visit to Alder Hey Children's Hospital on February 14, 2012, in Liverpool, England.
Prince William and Catherine help pack care packages for military children at the Mission Serve: Hiring Our Heroes event on July 10, 2011, in Culver City, California.
The Duchess of Cambridge stunned in a Jenny Packham gown on May 11 at an event hosted by the British Olympic Association. The teal number, complete with a lace back, is just one of her many noteworthy looks.
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