LONDON (AP) -- Britain doesn't have to wait any longer: Prince William's wife, Kate, is pregnant.

St. James's Palace made the announcement Monday, saying that the Duchess of Cambridge -- formerly Kate Middleton -- has a severe form of morning sickness and is currently in a London hospital. William was at his wife's side.

The news drew congratulations from around the world, with the hashtag "royalbaby" trending globally on Twitter.

The couple's first child will be third in line to the throne -- behind William and his father, Prince Charles -- leapfrogging the gregarious Prince Harry and possibly setting up the first scenario in which a 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 severe form of morning sickness that affects about 1 in 200 women and can lead to dehydration or worse if left untreated. They said she was expected to remain hospitalized for several days and would require a period of rest afterward.

Until Monday's announcement, the duchess had shown no signs of being with child. She was photographed just last week bounding across a field clad in black high-heeled boots as she played field hockey with students at her former school.

Still, speculation has swirled about when she and William would start a family from almost the moment they were wed on April 29, 2011, in a lavish ceremony at Westminster Abbey.

The attractive young couple is immensely popular -- with William's easy common touch reminding many of his mother, the late Princess Diana -- and their child is expected to play an important role in British national life for decades to come.

For months, Kate's every move has been scrutinized for clues about a possible pregnancy -- from each time she touched her stomach to whether her outfit choices hinted at a baby bump.

In September, tongues wagged over why she might be avoiding alcohol when the duchess opted to toast with a glass of ice water instead of champagne during a banquet in Singapore.

Last week, the rumor mill kicked into high gear when a beaming William accepted a baby outfit from a well-wisher that bore the phrase, "Daddy's little co-pilot."

"I'll keep that," he reportedly said.

The confirmation of Kate's pregnancy caps a jam-packed year of highs and lows for the young royals.

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.

Joe Little, managing editor of Majesty magazine, said the news ended a year that saw the royal family riding high in popular esteem after celebrations of Queen Elizabeth II's 60 years on the throne.

"People enjoyed the royal romance last year and now there's this. It's just a good news story amid all the doom and gloom," he said.

In the chilly night air at London's Camden market, concertgoers and shoppers seemed surprised by the news -- though all agreed that it had been widely anticipated.

"It feels a lot like a Christmas present for the nation!" said Ravian Van Den Hil, a Dutch student studying in London. "It makes me feel quite happy."

Others wondered why Britain continues to spend so much to support the royal family. "I don't think it's a good thing," said Stephen Jowitt as he strolled down Camden High Street. "It reinforces a class system."

The palace said the royal family was "delighted" by the news.

British Prime Minister David Cameron admitted he got a heads-up about the pregnancy, saying he found the news "quite difficult" to keep to himself and expressing confidence the young couple will make "absolutely brilliant parents."

The pregnancy comes after a 2011 decision by the leaders of Britain and the 15 Commonwealth nations endorsing new rules that give girls equal status with boys in the order of succession. Those changes make Kate's pregnancy all the more significant for the royal family, said Ingrid Seward, editor-in-chief of Majesty magazine.

"This is the first child who will be an heir to the throne, whatever sex they are," she said. "It's a new beginning."

Like Kate, William's mother, Diana, also reportedly suffered from morning sickness for months, and was the subject of constant media attention after she became pregnant just four months after her wedding to Prince Charles. "The whole world is watching my stomach," Diana once said.

According to Britain's Department of Health, severe morning sickness most often affects women early in their pregnancy, and is more common in young women, women who are pregnant for the first time and those expecting multiple babies.

Dr. Daghni Rajasingam, a spokeswoman for Britain's Royal College of Obstetricians and Gynaecologists, said women with severe symptoms -- including dehydration, dizziness and persistent vomiting -- need to be hospitalized for treatment, including being given fluids intravenously.

"However, this usually only means a few days in (the) hospital," she said in a statement. "The best advice for anyone suffering from (severe morning sickness) is to get plenty of rest and drink lots of fluid."

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Associated Press writers Jill Lawless, Paisley Dodds, Danica Kirka and AP Medical Writer Maria Cheng contributed to this report.