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

Catherine, Duchess of Cambridge and Prince William, Duke of Cambridge smile and wave to the crowds from the balcony of Cambridge Guildhall. [Photo credit: Getty Images | (Nov. 28, 2012]

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 currently 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. Palace officials said the duchess was hospitalized with hyperemesis gravidarum, a potentially dangerous type of morning sickness where vomiting is so severe no food or liquid can be kept down. They said she was expected to remain hospitalized for several days and would require a period of rest afterward.

Read more about Kates' pregnancy and the dangers of hyperemesis gravidarum;