The condition, which is marked by nausea and vomiting more severe than typical morning sickness, affects from 1% to 3% of pregnant women, according to the Hyperemesis Education and Research Foundation. It usually is found in women who are sensitive to the higher hormone levels that come with pregnancy.
Each month during a woman's menstrual cycle, the estrogen circulating through her body rises, Dr. James Liu told CNN in 2012, during the duchess' first pregnancy. Liu is now the chairman of the Department of Obstetrics and Gynecology, UH Cleveland Medical Center.
"But once a woman achieves pregnancy, the ovary at the beginning and later on the placenta makes much higher levels of estrogen and progesterone; and very high levels of estrogen in general can have an effect on a part of the brain called the chemoreceptor zone and it causes nausea," Liu said. During pregnancy, hormone levels can rise 100 to 1,000 times the amount of a normal menstrual cycle.
Extreme nausea and vomiting during pregnancy may occur if a woman is carrying more than one baby, according to the US National Institutes of Health. Another possible cause for higher hormone levels leading to excessive nausea is something called abnormal placentation, where the placenta overgrows, says Liu, adding that this happens only occasionally.
Usually, excessive vomiting is treated on an outpatient basis, where the mom-to-be is given anti-nausea medication, usually in pill form. But vomiting can lead to dehydration -- a real concern for a pregnant woman.
If she is very dehydrated, being in the hospital allows her to get fluids intravenously. Her electrolytes or blood minerals may also be somewhat abnormal, which can be corrected through an IV too.
The duchess is being cared for at Kensington Palace, a statement said, but she has been hospitalized previously for complications related to the condition.
The severity of nausea during pregnancy varies from woman to woman, said Liu, who is not involved in the duchess' care. Some may experience nausea but may not vomit, while others may be very sensitive, vomit excessively and are unable to keep fluids down.
The good news is that the nausea usually goes away by the end of the first trimester, said Liu. "The first 12 weeks of the pregnancy is the most common time we see this."