"Once you have had it once, the chances of having it again are much, much higher," Caitlin Dean, Vice Chair of Pregnancy Sickness Support, tells PEOPLE, saying that Kate had an 86 percent chance of developing the condition again.
Characterized by intense nausea and vomiting, Hyperemesis Gravidarum can lead to dehydration and weight loss. There is no cure.
Kate is being treated by doctors at Kensington Palace and skipped an event on Monday due to the illness. There is no word yet on whether the condition will affect Kate's upcoming engagements, including a two-day solo trip to Malta later this month.
"It's really, really hard," adds Dean, who's 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 tells 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, 32, 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