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... --snip-- Dr. 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...