St. James's Palace announced the pregnancy Monday, saying the Duchess of Cambridge, formerly known as Kate Middleton, has a severe form of morning sickness and is in a London hospital. William is at her side.
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 United Kingdom 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 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 that no food or liquid can be kept down. They said she was expected to remain hospitalized for several days and would need 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 Dr. Kecia Gaither, director of maternal fetal medicine at Brookdale University and Medical Center in Brooklyn.
The condition is thought to affect about one in 50 pregnant women. Gaither said less than 1 percent of women with the condition need to be hospitalized.
The news came just days after the duchess, on a royal appearance, played field hockey with children at her former school.
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.