Women who are underweight when they become pregnant have an increased risk of severe, potentially dangerous nausea and vomiting, a large study suggests.

Many women suffer “morning sickness” in the early months of pregnancy, but in a small percentage of cases, the nausea and vomiting progress to a condition known as hyperemesis gravidarum. Women with this condition have unrelenting, excessive nausea and vomiting that puts them at risk of malnutrition, dehydration and significant weight loss.

Because of the risks to the mother and fetus, severe vomiting may require hospitalization and IV fluids.

No one knows what causes the problem, but the new findings, published in the American Journal of Obstetrics & Gynecology, suggest that pre- pregnancy weight influences a woman’s risk.

Researchers found that among nearly 943,000 Swedish women who gave birth between 1992 and 2001, those who were underweight before pregnancy were 43 percent more likely than normal-weight women to be hospitalized for hyperemesis gravidarum.

In contrast, the study found, obese women had a 10 percent decreased risk compared with normal-weight women.

Underweight was defined as a body mass index (BMI) of less than 20, while obesity was defined as a BMI of 30 or higher. BMI is the ratio between weight and height.

Hyperemesis gravidarum was not common among women in the study, regardless of weight; overall, only 1 percent were hospitalized for the condition.

But underweight women and their obstetricians should be aware of their relatively higher risk, lead researcher Dr. Marie Cedergren, of the University of Linkoping in Sweden, told Reuters Health.

Women might be able to reduce the risk of excessive nausea and vomiting by eating small portions throughout the day, rather than large meals, Cedergren noted. Taking antiemetic drugs, which prevent vomiting, may also help keep morning sickness from progressing to hyperemesis, she added.

Underweight women in the study were also more likely than normal-weight women to have taken antiemetics early in pregnancy, while obese women were less likely.

It’s not clear why pre-pregnancy weight affects the risk of hyperemesis, Cedergren said. But she speculated that underweight women get a more severe form of the condition, more often necessitating hospitalization.

Another possibility, she and her colleagues note, is that doctors are quicker to hospitalize underweight women out of concern that they will rapidly become dehydrated and malnourished.

SOURCE: American Journal of Obstetrics & Gynecology, April 2008.