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.