It is not typical morning sickness, according to doctors. It is a rare and severe condition which can cause severe nausea and vomiting and lead to excessive weight loss and dehydration.
Patients are usually put on an IV to get fluids and nutrients.
Nearly all women have some nausea or vomiting , also known as "morning sickness", particularly during the first three months of pregnancy.
Doctors say once the illness is treated, it usually does not cause any complications for the infant or mother.
The causes are unknown but it can occur with twins or an abnormal pregnancy called a "molar pregnancy".
A statement from Saint James Palace said Middleton is expected to stay in the hospital for several days.
