Kate Middleton's pregnancy news comes with a bit of concern.

St. James's Palace confirmed to Us Weekly Monday, Dec. 3, that the Duchess of Cambridge is expecting her first child with husband Prince William**. "The Queen, The Duke of Edinburgh, The Prince of Wales, The Duchess of Cornwall and Prince Harry** and members of both families are delighted with the news," the statement read.

Unfortunately, it was also announced that William's pregnant wife, 30, has been hospitalized in Central London with Hyperemesis Gravidarum, an acute form of morning sickness that requires supplementary hydration and nutrients.

The statement said, "As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter."

So, what is Hypermesis Gravidarum and how common is it?

According to The National Organization for Rare Diseases (via Yahoo), the percentage of pregnant women who suffer from HG is estimated to be between .03% and 1%. While 70-80% of pregnant women experience some type of morning sickness, HG is characterized by severe nausea, vomiting, weight loss, dehydration, lightheadedness and fainting.

The National Center for Biotechnology Information reports that women suffering from HP usually have an ultrasound done to see if they are carrying twins and to check for a hydatidiform mole, a rare growth inside the uterus.

Treatment for HG can includes hospitalization for hydration, bed rest and medication. But helpher.org reports that it is typically resolved by 21 weeks.

After tying the knot in April 2011, Middleton and William, also 30, were eager to become parents. "Trying for a baby has been their priority," a royal confidant told Us. "They're both excited to be focusing on a new chapter in their lives."

This article originally appeared on Usmagazine.com: Pregnant Kate Middleton Hospitalized With Hyperemesis Gravidarum: What It Is