Dec. 3, 2012 -- Hyperemesis gravidarum, the reason newly pregnant Kate Middleton is in the hospital, is a rare but acute morning sickness that results in weight loss and accounts for about 2 percent of all morning sickness, doctors say.
The condition is sometimes associated with women having twins, experts said.
Women diagnosed with hyperemesis gravidarum have lost 5 percent of their pre-pregnancy weight, or 10 pounds, said Dr. Ashley Roman, a professor and OB/GYN at New York University Langone Medical Center.
It poses little danger to the tiny heir, doctors said.
"It's traditionally thought that nausea and vomiting is a sign of a healthy pregnancy," Roman said
Dr. Nancy Cossler, an OB/GYN at University Hospitals in Ohio said the condition does not cause loss of pregnancy or birth defects, but it can be a torture to endure.
"The biggest problem with this is how it interferes with your life," Cossler said. "Constantly feeling sick and puking is difficult."
Click here to read about other women with hyperemesis gravidarum.
Hyperemesis gravidarum is thought to be caused by higher levels of the pregnancy hormone, hCG, or human chorionic gonadotropin, Cossler said. Extra hCG can often be brought on by carrying more than one fetus, she said.
In other words, it could be a sign that Middleton is carrying twins. Although there's very little data on twins and hyperemesis gravidarum, one study showed that women carrying twins had a 7.5 percent higher risk of experiencing the acute morning sickness, Roman said.
The extreme morning sickness is usually diagnosed about nine weeks into the pregnancy, and in most cases resolves itself by 16 or 20 weeks, Roman said. In rare cases, it can last the whole pregnancy.
"As the pregnancy is in its very early stages, Her Royal Highness is expected to stay in hospital for several days and will require a period of rest thereafter," a statement from St. James Palace said. Prince William is at the hospital with Middleton, according to the Britain's Press Association.
Click here for photos of Kate through the years.
Roman said doctors 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, said Dr. Jessica Young, an OB/GYN at Vanderbilt University. But 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.
Hyperemesis gravidarum is somewhat mysterious because some expectant mothers have acute morning sickness during only one of their pregnancies, but have no morning sickness for subsequent pregnancies.
There is a chance that higher levels of hCG, which likely caused Middleton's nausea, could be a sign of a molar pregnancy instead of twins, Cossler said. This would mean Middleton is carrying only a benign growth in her uterus instead of a fetus, or she is carrying a fetus with abnormal DNA and a benign growth. Neither is considered a viable pregnancy.
However, Cossler said molar pregnancies become apparent early on, and doctors would already know whether Middleton had one.
"They would not have released this information," Cossler said of the birth announcement. "I'm certain that they have already eliminated both of those [types of molar pregnancies]."