Officials said the Duchess of Cambridge, aka Kate Middleton, was taken to a London hospital due to hyperemesis gravidarum, which is a severe morning sickness.
Medical experts say hyperemesis gravidarum is a very serious condition that causes severe vomiting and dehydration. It affects about 3 1/2 out of every 1,000 pregnant women. It can be so bad in some cases, that women vomit blood.
A palace source says Middleton is not yet in the twelfth week of her pregnancy. The royal family says her stay at the hospital will stretch for several days to make sure she stays hydrated and fed.
Maria Shriver also suffered from hyperemesis gravidarum. She tweeted about the news Monday.
"Wonderful news for Prince William & Kate Middleton. Kate has Hyperemesis. Had that with my last child. Not fun at all. Hang in there, Kate," she said.
The confirmation of her pregnancy follows intense speculation ever since the couple's lavish Westminster Abbey wedding last year. The news was greeted with congratuations from the rest of the royal family and their friends.
"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 palace said in a statement.
Whether boy or girl, the child will be next in line behind Prince William in succession to the throne, Cabinet Office officials have said. A child born to Prince William and the Duchess would be third in line to the throne and a great-grandchild of the Queen. The baby would bump Prince Harry down the line of succession to fourth place.
Even upon her release, Middleton will need an extended period of rest. So it may be a while before the royal couple has any public appearances. A due date has not been announced.
News of the royal pregnancy has spurred excitement in London and around the world.
"Very uplifting," said one London woman. "Because I think she has found the nation's heart."
The White House sent its well-wishes to the royal couple.
"On behalf of everyone here in the White House, beginning with the president and the first lady, we extend our congratulations to the Duke and Duchess of Cambridge on the welcome news this morning out of London that they are expecting their first child," said White House spokesman Jay Carney.
Hollywood is also reacting to the royal baby news. Khloe Kardashian tweeted her congratulations, and comedian and actor Zach Braff joked about where to find the right baby gifts.
"Where does one buy a tiny crown and scepter these days? Amazon?" he tweeted.
The Associated Press contributed to this report.