The Duchess of Cambridge is expecting her first child, but has also been admitted to hospital for treatment of nausea and vomiting related to her pregnancy.

In a statement posted Monday to Prince William and Catherine's website, the couple said they were "very pleased" to announce the pregnancy, which comes after weeks of speculation that an apparent heir to the British throne could be on its way.

Kate was admitted to King Edward VII Hospital in Central London on Monday afternoon with "hyperemesis gravidarum" -- a form of persistent morning sickness associated with pregnancy that can lead to dehydration. "Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter," the statement read.

The condition is thought to affect about one in 200 pregnant women, according to Britain's health department.

The announcement said only that Kate, 30, was in the "very early stages" of her pregnancy, but offered no details about when the baby is due.

"The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall and Prince Harry and members of both families are delighted with the news," the statement read.

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

Prime Minister Stephen Harper tweeted his congratulations shortly after the news broke, saying: "Laureen and I congratulate the Duke and Duchess of Cambridge on their wonderful news. #royalbaby"

British Prime Minister David Cameron also offered his best wishes over Twitter: "I'm delighted by the news that the Duke & Duchess of Cambridge are expecting a baby. They will make wonderful parents. #royalbaby"

Cameron later told reporters: “I’m sure everyone around the country will be celebrating with them tonight.”

Media speculation had been growing in recent weeks about a possible baby bump for the royal couple, who wed in April 2011 in a much-celebrated ceremony. The child -- no matter its gender -- will be third in line to the British throne after Prince Charles and father-to-be Prince William.

Royal commentator Victoria Arbiter hypothesized that the Duchess of Cambridge is very early on in her pregnancy, and said the couple likely wouldn't have yet gone public with the news if Kate didn't have to seek treatment in hospital -- a development which likely would have quickly leaked to the media.

Arbiter told CTV News Channel the British and international media can be expected to kick into high gear now that the news is official.

However, she said William has made it clear he will not put up with any inappropriate invasions of their privacy.

"In a way, I think that's why William was so adamant about filing a lawsuit when the topless photos of Kate were published. It was his way of saying to the paparazzi -- whom we haven't really seen in a frenzy since Diana -- that any kind of madness, stalking, crazy-style photography was not going to be tolerated."

This past year, the royal couple have travelled the world extensively as part of the Queen's Diamond Jubilee celebrations.

Joe Little, managing editor of Majesty magazine, told The Associated Press that the news wrapped up a year that saw the Royal Family riding high in popularity after celebrations of the Queen's 60 years on the throne.

"We're riding on a royal high at the moment at the end of the Diamond Jubilee year," he said. "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."

With files from The Associated Press