LONDON — While morning sickness in pregnant women is common, the problem Kate Middleton has been hospitalized with is not.

In a statement Monday, palace officials said she was hospitalized with hyperemesis gravidarum, a potentially dangerous type of morning sickness where vomiting is so severe no food or liquid can be kept down. Palace officials said the duchess was expected to remain hospitalized for several days and would require a period of rest afterwards.

"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 New York.

The condition is thought to affect about one in 50 pregnant women and tends to be more common in young women, women who are pregnant for the first time, those expecting multiple babies and in non-smokers. Gaither said that fewer than one percent of women with the condition need to be hospitalized.

Doctors aren't sure what causes it but suspect it could be linked to hormonal changes or nutritional problems.

Women admitted to the hospital with hyperemesis gravidarum are usually treated with nutritional supplements and given fluids intravenously to treat dehydration. Dr. Dagni Rajasingam, a spokeswoman for the Royal College of Obstetricians and Gynaecologists, said most women hospitalized with the condition are discharged within several days.

"It depends on how well the woman is keeping fluids down," she said.

If the problem is recognized and treated early, doctors say there are no long-term effects for either the mother or the child. Left untreated, the mother could be at risk of developing neurological problems — including seizures — or risk delivering the baby early.

Gaither said the condition usually subsides by the second trimester.

"The rest of the pregnancy could be entirely uneventful," she said, adding that pregnant women treated for the condition are usually advised to avoid fatty foods that could aggravate the problem.

Gaither said Middleton would probably be able to meet her usual royal obligations by her second trimester.

"She should be able to meet all her public obligations soon," she said, advising her to take her vitamins and ensure there are no other underlying health problems. "She should just be looking forward to having a healthy little plump person."