Kate Middleton, the Duchess of Cambridge, is expecting her third child. As with her previous two pregnancies, Middleton is again suffering from a rare form of morning sickness known as hyperemesis gravidarum.

The condition is a particularly acute form of morning sickness. Some level of nausea and vomiting affects 7 out of 10 pregnant women, according to the NHS. It is most common during the first trimester.

For some women the nausea can be extremely intense, leading to an inability to keep food down. It can lead to hospitalisation due to loss of fluids. Middleton was admitted to hospital for the condition in her previous pregnancies.

Vitamin B6 and the antiemetic Metoclopramide are sometimes prescribed for women experiencing hyperemesis gravidarum. However, medications for the condition need to be prescribed by a doctor during pregnancy to make sure they do not cause unexpected complications.

Hyperemesis gravidarum affects between 3 and 4 pregnant women per 1,000. Symptoms include:

  • Prolonged and severe nausea and vomiting, with some women being sick up to 50 times a day
  • Dehydration, due to the loss of fluids
  • Ketosis, which is a build-up of acidic compounds in the blood and urine as the body begins to break down its fat reserves
  • Weight loss
  • Low blood pressure, potentially leading to dizziness when standing

Milder cases of hyperemesis gravidarum can be treated with changes in diet, rest and antacids, according to the American Pregnancy Association. More serious cases require hospitalisation to ensure the safety of both mother and foetus.

The condition is thought to be caused by hormonal imbalance during pregnancy. It has been linked to levels of the hormone human chorionic gonadotropin (hCG), which is secreted by the placenta. hCG levels can double in early pregnancy every 2 to 3 days.

However, little is known about how this leads to intense sickness. There are no preventatives for the condition, but it can be managed with treatments including electrolytes and intravenous fluids.