Tim Draycott from the University of Bristol told WebMD that about 30 percent of women get some type of illness in pregnancy, while about one in 200 to one in 100 women get hyperemis gravidarum. Other estimates say it's one in 50 women, and tends to be more common in young women, women who are pregnant for the first time, and those expecting multiple babies, reports CBS.
Treatment usually involves nutritional supplements and fluids given intravenously to treat dehydration.
"As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in the hospital for several days and will require a period of rest thereafter," St. James's Palace said in a statement.
When treated early, experts say that there are no long-term effects for either the mother or the child from hypermesis gravidarum. But if left untreated, the mother could be at risk of developing neurological problems, or risk delivering the baby early.