Hyperemesis gravidarum, for which the Duchess of Cambridge was admitted to hospital, is an extreme form of morning sickness. Charlotte Bronte is reputed to have died of it in 1855, but nowadays doctors say it is a routine complication of pregnancy and easy to treat.

It is the name given to the situation when a pregnant woman cannot hold down fluids and becomes so dehydrated that she needs medical treatment, especially rehydration, to help her recover. "The symptoms include intractable vomiting, an inability to hold down food, dizziness or light-headedness, lethargy, headaches and sometimes actually fainting", said Dr Justin Clark, a consultant obstetrician and gynaecologist at Birmingham women's hospital.

A urine dipstick test analyses the ketones in the woman's urine to establish whether or not she is dehydrated. If she is, treatment includes rehydration with intravenous fluids, usually into a vein in the back of the hand, along with a dose of anti-emetic tablets to prevent further sickness. Some mothers-to-be also receive vitamin supplements, especially thiamine.

The woman usually feels better quite quickly, sometimes within hours. But the condition can recur up until the 12th or 14th week of pregnancy; it is not necessarily a one-off event. Sufferers usually spend no more than a day or two in hospital, and some women are treated as outpatients.

"We advise women who get it to eat small meals regularly, avoid fatty or spicy foods, drink plenty of fluids and take anti-sickness tablets regularly. Kate will probably have this sickness on and off throughout the first trimester, but most women get over it by then," said Clark, who is also a council member of the Royal College of Obstetricians and Gynaecologists.