The Duke and Duchess of Cambridge have announced that Kate is pregnant with their third child.

The Kensington Palace announcement also said that “as with her previous two pregnancies, The Duchess is suffering from Hyperemesis Gravidarum,” which is a form of morning sickness that is keeping her from attending her engagements.

CTV’s Royal Commentator, Richard Berthelsen, told CTV News Channel, that the sickness was likely the reason for the impromptu announcement of the pregnancy.

“The Duchess was meant to be at an engagement today, in London, at a children’s centre interestingly enough and she had to cancel,” he said. “Speculation would have started immediately.”

Although the disorder is extremely unpleasant for Kate and other women who suffer from it, there is likely nothing to worry about, as both previous pregnancies went without a hitch.

What is Hyperemesis Gravidarum?

Hyperemesis Gravidarum afflicts an estimated one to three per cent of pregnant women.

It tends to be more common in young women, women who are pregnant for the first time and women who are expecting multiple babies.

Symptoms typically occur in the first trimester of pregnancy and lasts much longer than regular morning sickness. Although some symptoms can improve at around 20 weeks there are cases where the expectant mother will experience sickness right up until the baby is born.

The condition can be "absolutely devastating," Dr. Roger Gadsby of Warwick University, who has studied the issue for decades, told The Associated Press. "Your life is on hold while the symptoms are present," he said.

It’s unclear what causes Hyperemesis Gravidarum but some doctors suspect it could be linked to hormonal changes or nutritional problems. There is also evidence that if someone has suffered from it in one pregnancy they are likely to get it again in subsequent pregnancies.

What are the symptoms of Hyperemesis Gravidarum?

Its symptoms include nausea, vomiting, weight loss and dehydration.

Extreme examples of Hyperemesis Gravidarum have seen women vomiting up to 50 times per day.

If women become too dehydrated there is a chance of getting ketosis, which is a serious condition that results in a build-up of acidic chemicals in the blood and urine.

How is hyperemesis gravidarum treated?

Dr. Mathias Gysler, an Ontario gynecologist, told CTV News Channel when the Duchess was pregnant with Charlotte, that when an expecting mother is experiencing symptoms of Hyperemesis Gravidarum it's important they stay hydrated.

"When you can't retain any fluid, your body requires water and fluids on an ongoing basis," he said.

Women with the condition are also advised to eat small meals often, to avoid any foods or smells that trigger symptoms and to consult their midwife or doctor if their symptoms do not subside.

Patients admitted to the hospital with Hyperemesis Gravidarum are typically treated with nutritional supplements and fluids administered intravenously. They may also be given anti-sickness medication. They may also be treated with shots of heparin, to thin their blood: pregnant women are at increased risk of developing blood clots in their legs, and being dehydrated further elevates the risk.

However, less than one per cent of women with the complication need to be hospitalized

Hyperemesis Gravidarum is unlikely to harm your baby, but can cause weight loss during pregnancy, so there is an increase in chance the baby will weigh less than expected.

If left untreated, patients could be at risk of developing neurological problems – including seizures - or delivering early.

The Duchess is currently “being cared for” at Kensington Palace, the official statement said.

With files The Associated Press