The happy news for the British monarchy is that Kate Middleton, the duchess of Cambridge, is with child—a potential heir to the throne. Sadly, the royal isn't having an easy go of her first pregnancy.

Middleton has been diagnosed with hyperemesis gravidarum (HG), which is a dangerous type of morning sickness. HG affects anywhere between 0.2 to 2 percent of all pregnant women in developed countries. The disease is associated with nausea and vomiting so severe that women can suffer from post-traumatic stress disorder as a result.

Scientific American spoke with Marlena Fejzo, a geneticist at the University of California, Los Angeles, David Geffen School of Medicine. Fejzo developed HG during her own pregnancy. Now she hopes to find the genes responsible for the most severe cases of HG. Fejzo explains what is known about this condition and its impact on pregnant women.

[An edited transcript of the interview follows.]

What is morning sickness?
General morning sickness is just a feeling of nausea and some vomiting in the first trimester of pregnancy. ("Morning") is a misnomer; there can be nausea and vomiting at any time of the day and even during the night.

How common is it?
Well, at least 70 percent of women have some sort of nausea and vomiting in the first trimester. It's a spectrum, so it can range from just feelings of nausea where you don't feel like eating something all the way to the extreme end of the spectrum, which is hyperemesis gravidarum.

What is hyperemesis gravidarum?
Hyperemesis is defined as nausea and vomiting that's persistent and prolonged and associated with a more than 5 percent weight loss. That's inability to perform your regular, daily routine due to nausea and vomiting.

There can be fainting. We have women who are vomiting so hard that they get rib fractures or their retina detaches. They can blow out their eardrums. They can get esophageal tears. We have women whose nails have fallen off due to malnutrition.

You can get Wernicke's encephalopathy, which is a thiamine deficiency that leads to a neurological disorder. It's generally seen in alcoholics because when you drink a lot you also have a problem with your brain absorbing thiamine. You get this inability to walk straight and mental confusion, and it can also end up in death of the baby and the mother.

How many people get this disease?
It's generally quoted between 0.2 and 2 percent of women. That could be an underestimate because it's often not diagnosed. Worldwide, I've seen it go all the way to 10 percent. There's (an academic) paper in Shanghai, China, that says 10 percent of women are hospitalized for HG.

How long does HG generally last?
Twenty-two percent of women have HG lasting through term. And most of them resolve around 20 weeks.

What are the consequences for the child? How dangerous is this?
Babies born from HG pregnancies are more likely to be low birth weight, small for gestational age and born prematurely.

Mothers who had severe nausea were more likely to have children with attention problems. Babies exposed to severe nausea in utero tend to have lower task persistence and attention problems. We found that adults who were exposed to HG in utero are more likely to have emotional and behavioral disorders—a 3.6-fold increased risk.

What causes HG? Why does it affect some so profoundly, but not others?
We are trying to find the cause of HG; we found that the number-one risk factor besides having a previous pregnancy (with HG) is having a sibling with HG. We know that there is likely to be a genetic component, and we're trying to find the predisposing genes. Once we find that, hopefully we can find the cause. It's likely to be something involved in hormone metabolism, but that's just speculation at this point.

If I get this in my first pregnancy, how likely am I to have it in the second and the third?
We've done a recurrence risk study: It's more than an 80 percent recurrence risk rate. Women who had changed partners, they had the same risk.

What's the economic cost?
There's more than $500 million in hospital charges in the U.S. yearly.* And emergency department visits are on the rise, too. It's up to over 220,000 emergency room visits in the U.S.

What steps can you take to prevent it?
We found that women tended to have it longer if they had allergies and if they had a special diet. And the special diets were lactose-free diets and veggie diets, so it may help to not have a veggie diet or lactose-free diet or kosher diet.

Tofu is high in estrogen, lactose-free milk is high in estrogen, so it could be that that's creating their symptoms.

But we don't know. There are a lot of questions and not very many answers.

*Correction (12/5/12): This sentence was edited after posting to correct the dollar amount originally stated.