All About Princess Kate's Morning Sickness: What Is Hyperemesis Gravidarum?

Kate, 35, has dealt with Hyperemesis Gravidarum, a form of acute morning sickness, during both of her previous pregnancies, as well.

The announcement from Kensington Palace Monday morning that Princess Kate is pregnant with baby no. 3 came with another, unfortunately familiar bit of news – that the Duchess of Cambridge is once again battling Hyperemesis Gravidarum during her pregnancy.

Kate, 35, has dealt with Hyperemesis Gravidarum, a form of acute morning sickness, during both of her previous pregnancies, as well.

“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 palace statement said.

“The Duchess is being cared for at Kensington Palace.”

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“Once you have had it once, the chances of having it again are much, much higher,” Caitlin Dean, Vice Chair of the U.K. advocacy group Pregnancy Sickness Support, told PEOPLE in 2014.

Dean added that the princess had an 86 percent chance of developing the condition again after experiencing it during her first pregnancy.

Characterized by intense nausea and vomiting, Hyperemesis Gravidarum can lead to dehydration and weight loss. There is no cure.

“It’s really, really hard,” said Dean, who suffered from the condition with each of her three pregnancies. “You’re happy to be pregnant, but at the same time you know you’ve got weeks or months of sickness ahead of you. It’s really bittersweet.”

“It might not be as bad as last time, and she will know how to deal with it better,” pregnancy expert and midwife Zita West told PEOPLE.

“From Kate’s point of view, it would be difficult planning engagements knowing you might have to cancel them,” Dean says. “She won’t want to let people down.”

So what kind of care plan should Kate have in place for dealing with the illness?

“You want to build up your vitamin B6,” Dean recommends. “We advocate taking it in advance, so you’re taking it in the very early stages. And working with your doctor on what medications worked last time and having a plan on when you start those.”
The pregnancy is sure to be hard on Prince William, 32, as well. “You don’t want to make your wife sick,” says Dean. “You want to have another child, but your wife has to go through this horrendous pregnancy.”

She suggests that William support Kate by “advocating for her … it’s about speaking up for her.”

• With reporting by SIMON PERRY and MONIQUE JESSEN