What is Kate Middleton's rare pregnancy condition?

Kate and William leaving the hospital with new daughter Charlotte. Photo / Getty Images

The rare pregnancy condition which the Duchess of Cambridge suffers from can be deadly, experts warn.

Hyperemesis Gravidarum (HG), which strikes just one per cent of pregnancies, is a complication that causes excessive nausea and vomiting.

Unlike regular morning sickness, it doesn't fade away with time and can leave some women bedbound as they are unable to keep food or drink down.

It is also considered to be the second leading cause of hospitalisation during pregnancy and can lead to dehydration - dangerous to both the mother and child.

If dehydration does strike, babies are at risk of deformities because the constant vomiting can deprive the woman's body of amniotic fluid - which the baby needs to thrive.

The condition may not clear up completely until the baby is born, although some symptoms may improve at around 20 weeks.

Some pregnant women are sick many times a day, which can have a negative effect on their daily life.

George and Charlotte will be joined by a new baby next year. Photo / Getty Images

Many mothers-to-be may lose up to 10 per cent of their body weight when they are supposed to be gaining about 1lb a week.

This can trigger a build-up of toxins in the blood or urine known as ketosis as the body tries to compensate for lack of food.

Exactly how many pregnant women get HG is not known as some cases may go unreported, but it's thought to be around one in every 100.

Signs and symptoms of HG include prolonged and severe nausea and vomiting, dehydration, low blood pressure and rapid weight loss.

Hospital treatment for these women is essential, as without intravenous feeding and fluids they are at risk of becoming dangerously dehydrated.

Until intravenous hydration was introduced in the 1950s, it was the leading cause of maternal death, Marlena Schoenberg Fejzo, a medicine researcher at the University of California, Los Angeles, previously told The Conversation.

But she warned that the condition is neither well-understood nor well-known, even with the "flurry of headlines" after it was announced Kate was struck down by it.

The condition can also affect the baby's development and there is a small risk it will be born prematurely or have a very low birth weight, which can be deadly as their bodies haven't fully developed.

In severe cases, the sickness can also trigger a miscarriage. It can also cause an abnormally fast heart rate in the mother, according to the NHS.

The condition is thought to be caused by elevated levels of "pregnancy hormone" HCG, or human chorionic gonadotropin, which increases after conception.

There is no cure but one of the most effective treatments is the drug Ondansetron, which is taken twice a day.

Risk factors for the condition include being overweight, having a family history of HG, and having twins or triplets.

Caitlin Dean, chairman of Pregnancy Sickness Support, previously said: "The severity of the condition should not be underestimated and the risks and complications of not treating it need to be appreciated."

The Duchess, 33, cancelled engagements while carrying Princess Charlotte as she battled HG. And in 2012, while expecting Prince George, she was admitted to hospital when HG left her severely dehydrated.

HOW THIS CONDITION COULD PREDICT YOUR BABY'S GENDER

Last year, researchers found those with extreme morning sickness may be less likely to give birth to sons.

According to New Scientist, one theory is the Trivers-Willard hypothesis which suggests when times are good, it is best to have a son.

However, in tough times, a daughter is safer for a mother to pass on her genes to as weaker males are less likely to succeed.

Given a third of pregnancies with HG result in miscarriage, this could help to explain why fewer boys are born to women with the condition, the study said.