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Kate Middleton's Illness Sometimes Associated With Twins

Arthur Edwards - WPA Pool/Getty Images(WASHINGTON) -- 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, Roman said.

"It's traditional thought that nausea and vomiting is a sign of a healthy pregnancy," Roman said. "Hyperemesis gravidarum in and of itself does not increase a risk of pregnancy loss, but it can be associated with multiple pregnancy problems like multiple gestations or molar gestations."

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, Rowan said.

But, if it's a sign of a molar gestation, Middleton could be carrying an abnormal fetus or no fetus at all.

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.

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.

Elevated pregnancy hormones and intestinal changes during pregnancy are thought to be contributing factors, Young 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.

Copyright 2012 ABC News Radio

ABC News Radio