Kate Middleton hospitalized for hyperemesis gravidarum: What is it?

Could Kate Middleton be pregnant with twins?

According to this backgrounder from the U.S. National Library of Medicine, one cause of hyperemesis gravidarum -- the severe form of morning sickness that has landed the expecting Duchess of Cambridge in King Edward VII Hospital in Central London -- is carrying multiples. That means twins, triplets or possibly more.

The duchess’ condition doesn’t necessarily signal that she and Prince William will be welcoming more than one prince or princess about nine months from now, of course. Hyperemesis gravidarum, or HG, arises in 0.3% to 2.0% of pregnancies, according to this 2008 report in Obstetrics and Gynecology Clinics of North America.

As many as 80% of pregnant women experience some type of morning sickness, according to the American Pregnancy Assn. Among them, about 60,000 women are hospitalized for hyperemesis gravidarum each year, according to studies cited by the health organization. Symptoms include severe nausea and vomiting, which can cause weight loss, malnutrition and electrolyte disturbance. Hospitalization allows patients to receive fluids and nutrition through an IV.

It’s not clear how far along Kate’s pregnancy is; the royal statement describes it as “in its very early stages.” The American Pregnancy Assn. says HG symptoms typically begin four to six weeks after conception. According to this overview from Medscape, symptoms resolve in 90% of patients by the 20th week of pregnancy.

Another cause of hyperemesis gravidarum is a hydatidiform mole, which forms inside the uterus during early pregnancy. These moles are rare, according to the National Library of Medicine, and are caused by “over-production of the tissue that is supposed to develop into the placenta.” Such cases are also known as “molar pregnancies,” and if a fetus does develop, it can’t survive.

Since the announcement from the royal family said Kate’s illness wasn’t serious, it seems a molar pregnancy is unlikely.

The HER Foundation, which describes itself as a grassroots network of HG survivors, released a statement of sympathy for Kate. “HG is so much more than morning sickness and its effects can be debilitating,” the group said. “We hope Kate knows she has tens of thousands of women around the world who understand what HG is and how it makes you feel physically and emotionally.”

Return to the Booster Shots blog.

Follow me on Twitter @LATkarenkaplan