WASHINGTON — Infant mortality in the United States fell to its lowest level ever in 2014, according to new federal data, as one of the biggest risk factors for infant deaths — preterm births — continued to fall.  Infant mortality rates in the United States have long been stubbornly high, particularly when compared with other rich countries. But after a recent peak in 2005, rates have been declining, down by 13 percent through 2013. In a report released by the Centers for Disease Control and Prevention on Wednesday, the rate declined again in 2014, down by 2.3 percent to a new low of 582.1 infant deaths per 100,000 live births, from 596.1 the year before.  “This is potentially the best news we’ve had yet,” said T. J. Mathews, a demographer at the C.D.C. who did not participate in the report but has written extensively about infant mortality.  Researchers could not say for sure what was driving the decline, but a fall in respiratory distress — when premature newborns have trouble breathing because their lungs are not fully developed — seems to have been part of it. Respiratory distress fell by 14 percent in 2014, compared to 2013.  Eugene R. Declercq, a health researcher at the Boston University School of Public Health, said there had been a push among doctors to stop inducing births or scheduling C-sections before 39 weeks of gestation unless there was a strong medical reason to do so. The practice of scheduling C-sections early and inducing labor early was believed to have been a potential contributor to infant mortality. “There’s been a conscious effort to change practices,” he said. “It has been one of the success stories.”  In a separate C.D.C. report released earlier this year, preterm births declined slightly, to 9.57 percent of births from 9.62 percent in 2013. And deliveries by cesarean section fell by 2 percent to 32.2 percent of deliveries, the lowest since 2007. Researchers debate whether C-sections are a risk factor for infant mortality.  It was not clear if the recent dip would improve the infant mortality ranking of the United States compared to other rich countries. Infant mortality in the United States was more than double the rates in Finland and Sweden in 2010, according to the C.D.C.  Dr. Steven Woolf, director of the Center on Society and Health at Virginia Commonwealth University, who has studied international comparisons of life expectancy, said he did not think the improvement in 2014 would change much for the international picture.  “Its good news that the infant mortality rate dropped last year, but it’s still much higher than the average of all the countries in the O.E.C.D.,” he said, referring to the Organization for Economic Cooperation and Development, a group of developed countries that serves as a close international comparison. In 2013, the rate in the United States was about 50 percent higher than the average rate for those countries, he said. The progress comes as the Obama-administration provides money for programs in which nurses and other professionals visit poor mothers most at risk of delivering a baby preterm, or of having an infant die. But it was not clear whether the improvement was related to such programs, which help a modest share of the population.  The report provided an early look at deaths for the entire American population, not just infants. Deaths from some of the most common causes declined. For example, the death rate from heart disease was down by 1.6 percent and down 1.2 percent for cancer. But rates for other ailments rose. The death rate for Alzheimer’s disease jumped by 8 percent in 2014 and the death rate for suicides was up by 3.2 percent.  A version of this article appears in print on December 9, 2015, on page A28 of the New York edition with the headline: U.S. Reports a New Low in Deaths of Infants. Order Reprints| Today's Paper|Subscribe
The rate of infant deaths fell more than 13 percent from 2005 to 2013 and by 2.3 percent in 2014, but the United States still has one of the highest rates among the richest nations.