Kate, the Duchess of Cambridge, is pregnant again. And according to the official announcement, the former Kate Middleton is suffering from the same condition that hospitalized her during her pregnancy with Prince George: hyperemesis gravidarum. For now, the duchess is in the care of doctors at Kensington Palace.

Although Kate’s first pregnancy drew significant attention to the condition, it looks like it’s time for a refresher.

What is hyperemesis gravidarum?

Hyperemesis gravidarum is morning sickness, but so much worse than what most pregnant women experience. Sufferers vomit. A lot. The nausea can be so severe that one begins to lose weight. Women are supposed to gain weight during pregnancy, so that in itself is worrying.

Beyond that, there are the possibilities of dehydration, and what the National Institutes of Health’s resource on the subject refers to as psychological or “social problems.” The duchess, as it turns out, cancelled an appearance in Oxford Monday with her husband. Sufferers, like the duchess, are often unable to go about their normal daily routine, whether that means going to work, taking care of kids, or repping the royal family. 

What causes it?

There’s no one good answer here, because doctors don’t precisely know. One of the risk factors, however, led to a lot of (incorrect!) speculation about the duchess’s pregnancy last time: “extreme nausea and vomiting during pregnancy,” the NIH’s resource guide explains, “can happen if you are pregnant with twins (or more babies).” There’s at least one other common possibility: she could have a hydatidiform mole, which appears in the uterus during early pregnancy.

Other risk factors associated with the condition include being overweight, having HG in a previous pregnancy, and being a first time mother.

Is it rare?

Most pregnant women experience some morning sickness. Severe morning sickness like the Kate’s, however, is more rare, though not unheard of. Scientific American wrote that somewhere between .2 and two percent of pregnant women in developed countries suffer from it. The numbers vary.  The condition does have a high recurrence rate: as many as 80 percent of women who have HG during their first pregnancy could have it for future pregnancies, SA added.

How long does it last?

Like the more common version of morning sickness, hyperemesis gravidarum hits in early pregnancy, and is almost always gone by the second half. Symptoms are usually the worst between two and 12 weeks, the NIH explains. It can, however, last longer.

The Daily Beast, reporting on ever-present royal rumors, digs in to how the couple might be in the very early stages of pregnancy despite the announcement. The duchess’s symptoms forced the couple to announce her pregnancy with George early as well, before the usual 12-week mark. Today’s announcement that she is pregnant again comes with no due date, so we’ll probably have to wait to learn more details.

Update: in a statement to reporters later on Monday, Prince William provided a little context the duration of his wife’s illness. “She’s feeling ok, it’s been a tricky few days, week or so. I’m going to go back and look after her now.” William added, “It’s great news, early days but I’m hoping that things settle down, she feels a bit better.” The couple is “thrilled” with the news, he said. 

What’s the treatment?

This part is tricky. Writer Jessica Grose, who experienced  hyperemesis gravidarum during her own pregnancy, wrote the following at Slate:

“Doctors don’t know how to help you. They don’t know why it happens. They don’t really know how to make you feel better. Two of the more common treatments for hyperemesis are antihistamines such as Benadryl, which doesn’t fix the nausea but does knock you out, and Zofran, which is a drug meant for cancer patients to help with the nausea and vomiting that is caused by chemotherapy and radiation.”

Otherwise, women suffering from HG are supposed to keep down as much fluids and foods as they can. That, obviously, is not easy.

Does hyperemesis gravidarum affect the fetus?

Possibly. A long-term study published in 2011 found that “children whose mothers suffered from HG while carrying them were 3.6 times more likely to suffer from anxiety, bipolar disorder and depression in adulthood.” Previous research has linked HG that lasts beyond the first trimester to learning disabilities in children.

Babies born of a mother with HG also have higher instances of a low birth weight and premature birth.

It should be noted, however, that these are possible risks and not necessarily characteristic of children born after HG in the mother. Prince George was born at a very healthy weight of 8 pounds, 6 ounces.

[this post has been updated]