The Duchess of Cambridge has been admitted to King Edward VII Hospital suffering from severe morning sickness.
Hyperemesis gravidarum is a rare condition which causes severe vomiting during pregnancy.
The severity of the vomiting can cause dehydration, weight loss and a build-up of toxins in the blood or urine called ketosis.
It affects 3.5 per 1,000 pregnant women and can cause women to vomit blood. Symptoms also include severe nausea, low blood pressure and fast heart rate, headaches, lethargy or confusion.
Hyperemesis gravidarum is treated by giving women fluids intravenously and by anti-sickness tablets.
"In very simple terms hyperemesis means vomiting a lot and gravidarum means in pregnancy," said consultant obstetrician Daghni Rajasingam, spokeswoman for the Royal College of Obstetricians and Gynaecologists.
"The diagnosis is given when women cannot keep food or fluid down because she has severe vomiting. The women who are vomiting pretty much constantly, that cannot keep any nutrients down, they need to be admitted to hospital."
She said the length of stay in hospital depends on each patient but many women are discharged in a matter of days.
"It depends on how well the woman is keeping fluids down," Ms Rajasingam added.
She said the condition is thought to be caused by elevated levels of the "pregnancy hormone" hCG. The body begins to produce human chorionic gonadotrophin (hCG) after conception.