(LONDON) -- Kate Middleton is pregnant.
The most eagerly awaited pregnancy was announced Monday by St. James Palace on behalf of Middleton and her husband Prince William.
The child, whether boy or girl, will eventually be heir to the British throne, according to new legislation awaiting final approval.
The duchess was admitted to King Edward VII Hospital Monday in central London with hyperemesis gravidarum, an acute morning sickness which requires supplementary hydration and nutrients, the palace said.
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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.
"Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting a baby," the palace said in a statement Monday. "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."
The baby will be the queen's third great-grandchild.
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Robert Lacey, author of the definitive book Majesty said, "The British public and indeed the whole world will be delighted for the same, it keeps the monarchy going. The royal wedding brought a magic back to the monarchy and people are fascinated by William and Catherine."
Last year, the heads of 16 Commonwealth countries agreed to a change in the rules of succession so that first-born children of either gender can take the throne. Queen Elizabeth II was only eligible to be monarch because her father had no male children. The British Parliament must still amend existing law to make the succession change official.
"Put simply, if the Duke and Duchess of Cambridge were to have a little girl, that girl would one day be our queen," British Prime Minister David Cameron told reporters in 2011.
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Royal babies have typically been born within one year of marriage. Princess Diana gave birth to William just 11 months after her wedding and the queen gave birth to Prince Charles six days before her first wedding anniversary.
Prince William and Kate were married on April 29, 2011.
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