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 . Acute morning sickness : What is it ? 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 . Read more : Who can inherit the British throne ? `` 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 . Former PM : William and Kate reinvigorated monarchy . 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 . In photos : Duchess of Cambridge at 30 . 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 .
Catherine has hyperemesis gravidarum , more severe than typical morning sickness . The baby would be next in line to the British throne after William . The pregnancy is in ` ` its very early stages , '' the doctor says . The birth would bring the first grandchild for King Charles and the late Queen .