High rates of premature birth are the main reason the United States has higher infant mortality than do many other rich countries, government researchers reported Tuesday in their first detailed analysis of a longstanding problem. In Sweden, for instance, 6.3 percent of births were premature, compared with 12.4 percent in the United States in 2005, the latest year for which international rankings are available. Infant mortality also differed markedly: For every 1,000 births in the United States, 6.9 infants died before they turned 1, compared with 2.4 in Sweden. Twenty-nine other countries also had lower rates. If the United States could match Sweden's prematurity rate, the new report said, 'nearly 8,000 infant deaths would be averted each year, and the U.S. infant mortality rate would be one-third lower.' The first author of the report, Marian F. MacDorman, a statistician at the National Center for Health Statistics, said in an interview that the strong role prematurity played came as a surprise to her. Dr. Alan R. Fleischman, medical director for the March of Dimes, said the new report was 'an indictment of the U.S. health care system' and the poor job it had done in taking care of women and children. The report, Fleischman added, 'puts together two very important issues, both of which we knew about but hadn't linked tightly.' Infant mortality is widely used as a way to gauge the health of a nation, and the relatively high rates in the United States have long dismayed health officials. Most European countries -- as well as Australia, Canada, Hong Kong, Israel, Japan, New Zealand and Singapore -- have lower rates of infant death than the United States. Premature infants in the United States are more likely to survive than those elsewhere. Yet they are still more likely to die than full-term babies, and the sheer numbers born prematurely in the United States -- more than 540,000 per year -- drive up infant mortality. The high levels of prematurity in the United States have various causes. Fleischman said the smallest, earliest and most fragile babies were often born to poor and minority women who lacked health care and social support. The highest rates of infant mortality occur in non-Hispanic black, American Indian, Alaska Native and Puerto Rican women. But other minorities have some of the lowest infant mortality rates in the United States: Asian and Pacific Islanders, Central and South Americans, Mexicans and Cubans. When it comes to prematurity, infertility treatments -- drugs that stimulate ovulation and procedures that implant more than one embryo in the uterus -- also play a role by raising the odds of twins or higher multiples, which have an increased risk of being born too soon. Professional groups for fertility doctors recommend limiting the number of embryos transferred to avoid multiple births, but ultimately doctors and patients make their own decisions. MacDorman said that because most insurance in the United States did not cover infertility treatments, some patients chose to transfer multiple eggs in hopes that doing so would increase the odds of pregnancy and reduce expensive procedures. 'In Europe, they may have been more successful in limiting the number of embryos transferred,' MacDorman said, 'because there is more national health insurance and people don't have to pay out of pocket.' Another factor in the United States, she said, is the increasing use of Caesarean sections and labor-inducing drugs to deliver babies early. The American College of Obstetricians and Gynecologists has guidelines stating that babies should not be delivered before 39 weeks without a medical reason, but doctors may be declaring a medical need more quickly than they did in the past. 'I don't think there are doctors doing preterm Caesarean sections or inductions without some indications,' MacDorman said, 'but there sort of has been this shift in the culture. Fifteen or 20 years ago, if a woman had high blood pressure or diabetes, she would be put in the hospital, and they would try to wait it out. It was called expectant management. 'Now I think there's more of a tendency to take the baby out early if there's any question at all.' These births -- called 'late preterm,' which occur after 34 to 37 weeks of pregnancy, instead of the normal 38 to 42 weeks -- are the fastest-growing subgroup of premature births. A late preterm baby's risk of dying is about three times that of a full-term infant. But late preterm babies are still far more likely to survive than very premature ones, and the very early babies account for much of the death rate, Fleischman said. Taking care of women's illnesses and problems like drinking, drug use and smoking before and during pregnancy can help prevent prematurity, he said, adding that a state program in Kentucky to provide home visits by nurses to poor women during pregnancy had decreased preterm births. MacDorman said prematurity was not the only factor behind infant mortality in the United States. She said full-term babies in this country also had higher death rates than those in Europe from sudden infant death syndrome, accidents, assaults and homicides. 