"Obviously we're thrilled," Prince William said today at an official event in Oxford, England. "It's great news."
Kate, 32, was scheduled to travel to Oxford with William but is instead being treated by doctors at Kensington Palace today, royal officials said.
“She's feeling OK," Prince William said. "[It's the] early days but I hope things settle down and she feels a bit better."
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The duchess is not yet 12 weeks pregnant, Palace officials told ABC News.
The Palace would likely not have released news of Kate’s pregnancy for another five to six weeks had she been healthy enough to attend public events, ABC News royal contributor Victoria Murphy, who is in Oxford covering the royal event, said.
“It’s not something that they would have wanted to do,” Murphy said. “They had to announce it today because she was due to attend here today and she couldn’t be here and they knew that there would be huge questions and speculation about why she wasn’t here.”
Kate was not yet 12 weeks pregnant in December 2012 when the Palace announced her pregnancy with Prince George. In that case, the couple decided to go public with the news of the pregnancy after the duchess was admitted to the hospital for hyperemesis gravidarum, a Palace source said at the time.
Hyperemesis gravidarum is acute morning sickness that requires supplementary hydration and nutrients.
“This is morning sickness like a hurricane is a little bit of rain,” said ABC News’ medical contributor Dr. Jennifer Ashton, an ob-gyn who has not treated the duchess. "It's really not morning sickness."
“Typically this resolves around mid-pregnancy but in some extreme cases it can last the entire nine months," she added.
Kate was due to visit Malta in less than two weeks, her first solo royal engagement abroad. But given her morning sickness, Palace officials will determine whether she's able to attend.
“While the couple is obviously very excited, they are very cautiously excited,” royal contributor Murphy said.
The baby, expected to be born in the spring, will be a younger sibling to 1-year-old Prince George. He or she will be fourth in line to the British throne after grandfather Prince Charles, father Prince William and big brother George.
It is playfully called "an heir and a spare," with George in direct line to the throne and the new baby his alternate. Second-born royal babies can end up as monarch. Britain’s last King, George VI, acceded to the throne in 1936 when his older brother Edward VII abdicated.
British Prime Minister David Cameron expressed excitement about the announcement.
“Many congratulations to the Duke and Duchess of Cambridge. I’m delighted by the happy news that they’re expecting another baby,” he wrote on Twitter.