The news that William and Kate will be welcoming a new addition to their family and a sibling for Prince George and Princess Charlotte has been slightly marred over concerns for the Duchess after she had to pull out of a public engagement yesterday due to severe morning sickness.

The Duchess, who is less than 12 weeks pregnant, is currently being cared for at Kensington Palace but she has previously been admitted to hospital for hyperemesis gravidarum (HG) – a rare condition that can lead to dehydration, weight loss and a build-up of toxins in the blood or urine called ketosis.

With both her previous pregnancies, Kate suffered from the rare condition which also causes severe vomiting.

Experts warn the condition often gets worse with subsequent pregnancies, but those who have experienced it before can be put on medication as soon as they get a positive pregnancy test.

When expecting Prince George in 2012, Kate spent three nights in King Edward VII’s Hospital in central London before being discharged.

With Princess Charlotte, the Duchess was cared for at home, as is now the case with her third pregnancy.

Nicola James, trustee of the charity Pregnancy Sickness Support, said: “It’s highly likely that if you have had it before, you will have it again.

“It’s generally worse in subsequent pregnancies. It starts earlier and is usually more severe.

“But if it was treated well last time and you can get things in place, you might be all right.

“Some women even go as far as seeing their consultant and being put on medication as soon as they get a positive pregnancy test.”

Hyperemesis gravidarum (HG) affects 3.5 in every 1,000 pregnant women and can cause them to vomit blood.

It can be treated with anti-nausea tablets and by giving fluids intravenously.

Normal morning sickness (known medically as nausea and vomiting in pregnancy, or NVP) affects around 70 per cent of pregnant woman, and causes some degree of vomiting and discomfort commonly during the first trimester – 12 weeks – of pregnancy.

But those suffering from HG can be constantly sick morning, noon and night and unable to keep any food or drink down.

Other symptoms include dehydration, low blood pressure, tiredness, dizziness and weight loss, and HG can go on for much longer too, sometimes, although rarely, for the entire pregnancy.

Wrexham GP and Leader medical columnist Dr Peter Saul said: “Calling it morning sickness is wrong because, although symptoms may be worse first thing, it affects many women during all waking hours.

“Probably around a third of the pregnant women I see have this to some extent but it is usually only mild and lasts a few weeks. Most dont need strong drugs and hardly any need admission to hospital.

“The usual outcome is that things settle on their own and the women go on to have normal pregnancies. In just a few cases symptoms can last much longer than the first third of pregnancy and they can be very severe.

“Apart from being unpleasant for the woman, the danger is that vomiting can lead to dehydration, weight loss and a biochemical imbalance that can put the health of both the mother and baby at risk.

“Sometimes medication needs to be used to suppress symptoms – but giving drugs to women in early pregnancy is fraught with danger.”

Despite this danger, there are medications that can be used in pregnancy, including the first 12 weeks, to help improve the symptoms of HG.

These include anti-sickness (anti-emetic) drugs, vitamins (B6 and B12) and steroids, or combinations of these.

“Years ago a wonder drug was introduced for this condition and it did seem to help but its name was Thalidomide and we all know the terrible side effects,” continued Dr Saul.

“Nowadays we use tried and tested medications in as small doses as possible. Intravenous fluids may be given to keep the stomach rested and to allow any fluid of chemical imbalance to be corrected.”

It is still unclear exactly what causes HT, but it may be linked to changing hormones in the body during pregnancy.

It may also run in families, according to the NHS. Those who’ve had it during one pregnancy are likely to get it in subsequent pregnancies, so the NHS urges people to plan ahead.

“We still dont fully understand why some women get sickness but it’s probably related to the hormonal changes in pregnancy as well as some dietary factors,” added Dr Saul.

“Women who are prone to travel sickness, are having their first baby or are having twins seem more at risk.

“If I was advising Kate, I’d suggest she rest as much as possible and get up slowly in the morning, take small frequent meals and make sure they are as bland as possible, distract yourself and wear comfortable loose fitting clothes.

“Using ginger can help, as well as acupressure bands. I also sometimes use homeopathy in the form of Ipecac 6C.”