Kate Middleton's Hyperemesis Gravidarum Explained

— -- Once again, the royal family was forced to announce Kate Middleton’s pregnancy early, thanks to a rare but severe form of morning sickness.

Middleton has hyperemesis gravidarum, which is diagnosed when a pregnant woman loses more than 10 pounds due to extreme and persistent nausea and vomiting, according to the Cleveland Clinic.

News of Middleton’s first pregnancy broke in December 2012 when she was admitted to King Edward VII Hospital in London with the same illness. She gave birth to her son George the following July.

The duchess is not yet 12 weeks into her second pregnancy, royal officials told ABC News, and doctors are treating her at Kensington Palace. They said she “may require supplementary hydration, medication and nutrients.”

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Although every pregnancy is different, having hyperemesis gravidarum during one pregnancy may make a woman more likely to have it in subsequent pregnancies, according to Dr. Jennifer Ashton, a practicing OB/GYN and senior medical contributor for ABC News. And it may be worse the second time around.

“Sometimes subsequent pregnancies have it starting sooner, lasting longer and being more severe,” said Ashton, adding that the debilitating pregnancy disorder is morning sickness “like a hurricane is a little bit of rain.”

Hyperemesis gravidarum occurs in between 1 and 2 percent of all pregnant women and poses little danger to the tiny heir when properly treated, but it can be torturous to endure, according to Dr. Nancy Cossler, an OB/GYN at University Hospitals in Ohio.

"The biggest problem with this is how it interferes with your life," Cossler said. "Constantly feeling sick and puking is difficult."

Although the cause of hyperemesis gravidarum is unknown, it may be related to high levels of the pregnancy hormone, hCG, or human chorionic gonadotropin, Cossler said.

The disorder is usually diagnosed about nine weeks into the pregnancy, and in most cases resolves itself by 16 or 20 weeks, according to Dr. Ashley Roman, a professor and OB/GYN at New York University Langone Medical Center. In rare cases, it can last the whole pregnancy.

Roman said doctors often prescribe vitamins and ginger capsules at first. If that doesn't stop the vomiting, they will prescribe antihistamines and stronger anti-nausea medications.

Women with hyperemesis gravidarum are also treated with fluids, according to Dr. Jessica Young, an OB/GYN at Vanderbilt University. If left untreated, a pregnant woman who is severely dehydrated for a long period of time could die "just like any person," Young said.

In extreme cases in which the woman is losing weight and unable to eat, doctors will treat her with intravenous nutrition, Young said.

Hospital stays can vary, and women will often have to be admitted more than once before the condition passes, doctors said.