When the Duke and Duchess of Cambridge announced to the world that they were expecting their third child Royal watchers were overjoyed!

The exciting birth of a third royal baby means more big changes for one of the most celebrated families in the world.

Little Prince George and his younger sister, Princess Charlotte, will soon welcome another sibling into the family and the Royal line can can count another heir to strengthen the lines of succession.

But the joyous news comes tinged with a very real fear...

While we can only imagine how excited the excited the royal couple must be to welcome their new son or daughter into the world, there is no doubt that the Duchess of Cambridge has something very worrying on her mind.

During her past two pregnancies with Charlotte and George, the Duchess suffered from an extreme form of morning sickness.

The Palace has recently confirmed that she is again suffering from the same form of extreme nausea and vomiting. She was so ill that she was unable to attend George's first day of school.

"As with her previous two pregnancies, the Duchess is suffering from Hyperemesis Gravidarum," said the press release.

Hyperemesis Gravidarum - What is it?

According to pediatric expert, Dr. Tamara Bugembe, the condition is the severest form of nausea and vomiting experienced during pregnancy.

"The nausea and vomiting can be so severe that it starts to cause complications such as dehydration, an imbalance of the body's salts and weight loss of more than five per cent of the mother's pre-pregnancy weight," she explains in the Evening Standard.

Women with hyperemesis gravidarum (HG) can vomit up to 30 times a day in the first few months of their pregnancies. This much sickness doesn't give the body enough time to recover and restore its fluids.

The condition affects about 3.5 out of 1, 000 pregnant women. Severe cases can cause women to vomit blood, low blood pressure, fast heart rate, headaches, lethargy and confusion.

They can also experience dehydration, weight loss and a build-up of toxins in the blood or urine called ketosis.

When discussing Kate's most recent pregnancy, Prince William said, "it was a bit anxious to start with, (but they can) start celebrating this week."

"There's not much sleep going on at the moment," he added.

Before modern medicine, HG was a common cause of death in pregnancy. Thankfully, the Duchess of Cambridge is getting the best medical treatment, likely including intravenous fluids and anti-sickness tablets.

Early guesses estimate that Royal Baby #3 will be born sometime in April or May.

[h/t Evening Standard / Forbes/ Mirror]