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.
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.
"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.
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.
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.