In two new studies, researchers have pinpointed the most common causes of stillbirths and have found that known risk factors explain just a small minority of cases.

According to the Centers for Disease Control and Prevention, about 26,000 fetal deaths at 20 weeks’ gestation or later occur annually in the United States, a rate higher than in many other developed countries. The reasons are unclear.

Both studies appeared last month in The Journal of the American Medical Association.

In the first report, a team of scientists at several sites examined 512 babies born with no signs of life at 20 weeks’ gestation or later. The sample was diverse in ethnicity, race and geography, roughly approximating the population of the United States.

The researchers considered a number of possible causes, including placental abnormalities, infection in the mother or baby, and maternal medical conditions. They found a possible or probable cause of death in about 76 percent of the stillbirths.

Deaths during labor accounted for about 29 percent of cases. In this group, infections were more common, and genetic abnormalities less so, than in babies who died before labor.

Placental disease accounted for 24 percent of stillbirths over all. About 13 percent of deaths were caused by infection, almost 14 percent by genetic abnormalities, and more than 10 percent by umbilical cord conditions that prevented sufficient blood flow.

Hypertension and other maternal medical complications together accounted for 17 percent of the deaths.

There were significant racial and ethnic variations. African-American women were more likely to have a stillbirth, and it was more likely to occur early in pregnancy and after the onset of labor. Infections were also more common in black women. Umbilical cord abnormalities, on the other hand, were more frequent in white and Hispanic women.

Race and ethnicity were not associated with deaths that occurred before the onset of labor or those that happened after 24 weeks’ gestation.

An author of the paper, Dr. Bob Silver, a professor of obstetrics and gynecology at the University of Utah, said doctors should push to learn the cause of the stillbirth with an autopsy, even if patients are at first hesitant to consent to one.

“A lot of families are reluctant to do a postmortem, but almost invariably, finding a cause of death helps them heal emotionally,” he said. “And if they want another baby, it can help prevent the same thing from happening again.”

In the second study, researchers tried to determine which risk factors apparent at the start of pregnancy could predict stillbirth, and in particular which could account for the racial disparities. They compared 614 stillbirths with 1,816 normal deliveries.

Several factors were independently associated with stillbirth: diabetes, age over 40, AB blood type, a history of drug use or smoking, obesityhypertension, previous stillbirth and not living with a partner. Some were more frequent among African-Americans, including previous stillbirth, obesity, diabetes and chronic hypertension.

But all of these factors together, easily identifiable at the start of pregnancy, explained only about a fifth of the risk for stillbirth, and many questions remain. For example, black women are twice as likely to have a stillbirth as women of other races or ethnicities, but the disparity in risk occurs almost entirely at less than 24 weeks’ gestation. The reasons for this are unknown.

Dr. George R. Saade, an author of the study, said the problem of these racial differences had not received wide attention. “This study highlights a major problem,” he said, “and more research will be needed to identify the causes of the racial disparities and find additional risk factors that may appear during pregnancy.” Dr. Saade is chief of the division of maternal-fetal medicine at the University of Texas Medical Branch at Galveston.

Obviously risk factors like race and age cannot be altered. Still, Dr. Silver said, “it makes sense to modify the risk factors that can be modified, and patients should be reassured that even with risk factors, most pregnancies result in healthy live births.”