The Duchess of Cambridge, pregnant with her second child, is being treated for a form of morning sickness called hyperemesis gravidarum, royal officials confirmed Monday.

Clarence House, the office for the couple, made the announcement before the 12-week stage of pregnancy as the duchess had started experiencing symptoms.

During her first pregnancy, the duchess was briefly hospitalized for the same condition. She later recovered and went on to deliver her first child, Prince George, without further complications.

Here are a few facts about the extreme form of morning sickness:

  • Hyperemesis gravidarum afflicts an estimated 0.5 to two per cent of pregnant women
  • Its symptoms include nausea, vomiting and dehydration
  • It typically occurs in the first trimester of pregnancy and lasts much longer than regular morning sickness
  • It tends to be more common in young women, women who are pregnant for the first time and women who are expecting multiple babies
  • Fewer than one per cent of women with the complication need to be hospitalized
  • Patients in hospital with hyperemesis gravidarum are typically treated with nutritional supplements and fluids administered intravenously
  • If left untreated, patients could be at risk of developing neurological problems or delivering early

Dr. Mathias Gysler, an Ontario gynecologist, told CTV News channel that when a 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.

He added that although the condition usually abates after the first trimester, he and his colleagues have seen cases that have carried on throughout the entire pregnancy.

When the duchess was treated for the condition in her first pregnancy, Dr. Kecia Gaither, director of maternal fetal medicine at New York's Brookdale University and Medical Center, told The Associated Press that doctors become concerned if the symptoms don't subside.

"It's not unusual for pregnant women to get morning sickness, but when it gets to the point where you're dehydrated, losing weight or vomiting so much you begin to build up (toxic) products in your blood, that's a concern," Gaither said.

With files from The Associated Press