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

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 UK 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.

"It's not unusual for pregnant women to get morning sickness, but when it gets to the point where you're dehydrated, losing weight or vomiting so much you begin to build up (toxic) products in your blood, that's a concern," said Kecia Gaither, director of maternal fetal medicine at Brookdale University and Medical Center in New York.

The condition is thought to affect about one in 50 pregnant women but Gaither said less than one percent of women with the condition need to be hospitalized.

The news came just days after the duchess, on a royal appearance, was playing field hockey with schoolchildren at her former school.

Not only are the attractive young couple popular with William's easy common touch reminding many of his mother, the late Diana but their child is expected to play an important role in British national life for decades to come.

William is second in line to the throne after his father, Prince Charles, so the couple's first child would normally become a monarch eventually.

The confirmation of Kate's pregnancy caps a jam-packed year of highs and lows for the young royals, who were married in a lavish ceremony at Westminster Abbey last year.

They have traveled the world extensively as part of Queen Elizabeth II's Diamond Jubilee celebrations and weathered the embarrassment of a nude photos scandal, after a tabloid published topless images of the duchess.