What is Hyperemesis Gravidarum? Duchess Kate Suffering From Severe Morning Sickness With Baby No. 3

The duchess is pregnant with her third child with Prince William, it was announced Monday.

Prince William and Princess Kate are expecting baby number three!

Kensington Palace announced the surprise news Monday morning in a statement.

Read: Duchess Kate Released From Hospital

“Their Royal Highnesses The Duke and Duchess of Cambridge are very pleased to announce that The Duchess of Cambridge is expecting their third child,” a statement from Kensington Palace said. “The Queen and members of both families are delighted with the news.””

The couple, who already have two children, 4-year-old George and 2-year-old Charlotte, reportedly announced the news prematurely because Kate, 35, suffers from severe morning sickness, also known as Hyperemesis gravidarum. She was forced to cancel a scheduled Monday morning appearance due to her condition.

“As with her previous two pregnancies, The Duchess is suffering from Hyperemesis gravidarum. Her Royal Highness will no longer carry out her planned engagement at the Hornsey Road Children’s Centre in London today,” the statement said.

Hyperemesis gravidarum affects about one in every 200 pregnancies and results in severe nausea, dehydration, and low blood pressure, among other things.

Dr. Jennifer Ashton told Good Morning America that the condition can be dangerous. 

“We’re talking about the potential for severe malnutrition, electrolyte abnormalities, sodium and in some cases organ failure… with HG you’re talking about vomiting that could be every hour or every other hour," she said.

Kate is being cared for at Kensington Palace. She is reportedly less than three months pregnant.

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With Kate’s previous two pregnancies, the couple also announced before the three-month mark because of the Duchess' condition. She was hospitalized due to Hyperemesis gravidarum while she was a few weeks pregnant with Prince George in 2012.

The birth will bump Prince Harry down to sixth in line to the throne.

Watch: Parents Say Sonogram Shows Jesus' Face in Womb With Baby