Britain's Prince William and wife Catherine are expecting their first child, but her severe morning sickness is attracting the most questions Monday.
The Duchess of Windsor is suffering from Hyperemesis gravidarum, according to St. James's Palace.
Not all pregnant women have morning sickness. Up to 2% of women who do have morning sickness may have severe vomiting, according to the American College of Obstetrics and Gynecology.
The college advises women who can't keep anything down for 24 hours of more to call their doctors. In severe cases, such as Middleton's, women can become deyhydrated due to lack of fluids. These women may need to be hospitalized for a day or two so doctors can administer intravenous fluids.
The palace said that since the pregnancy is in its "very early stages," the 30-year-old duchess is expected to stay in the hospital for several days and will require a period of rest afterward.
Prince William is second in line to the throne after his father, Prince Charles, so the couple's first child would normally become monarch in due course. They married 19 months ago. The Palace did not say how long she has been pregnant.