LONDON — The most widely anticipated pregnancy since Princess Diana’s in 1981 is official: Prince William’s wife, Kate, is pregnant.

St. James’s Palace announced the pregnancy Monday, saying that the Duchess of Cambridge — formerly known as Kate Middleton — has a severe form of morning sickness and is in a London hospital. William is at his wife’s side.

News of the pregnancy drew congratulations from across the world, with the hashtag “royalbaby” trending globally on Twitter.

The couple’s first child will be third in line to take the throne — leapfrogging the gregarious Prince Harry and possibly setting up the first scenario in which a U.K. female heir could benefit from new gender rules about succession.

The palace would not say how far along the 30-year-old duchess is, only that she has not yet reached the 12-week mark.