Infant mortality has been falling in the United States for the last decade, with declines in the three leading causes: birth defects, prematurity, and sudden infant death syndrome, federal researchers reported Tuesday.

For every 1,000 babies born in the U.S., the number who died before their first birthday fell from 6.9 in 2005 to 5.8 in 2014, a 15 percent drop, according to the report from the U.S. Centers for Disease Control and Prevention.

The death rates fell in 33 states, including Pennsylvania and New Jersey, and reached new lows for all racial and Hispanic subgroups except for American Indian and native Alaskans. Black infant death rates fell 20 percent, to 10.9 per 1,000 births -- but remained more than double the white rate of 4.9 per 1,000.

The trends are good news for the U.S., which has much higher infant mortality than other affluent countries. One reason, studies have found, is that the U.S. records a higher proportion of extremely premature babies -- who have almost no chance of survival -- as live births. But studies also show that poor and disadvantaged mothers in the U.S. have higher infant mortality rates than wealthier ones.

"Infant mortality is considered a basic measure of public health for countries around the world," wrote the CDC team, led by researcher T.J. Mathews. "Over the years, many efforts have been made to understand and lower infant mortality."

While the report did not discuss those efforts, child health experts cited them to explain the encouraging trends.

Sara Kinsman, the Philadelphia Department of Health's director of maternal, child, and family health, noted that several home-visiting programs are aimed at reducing racial and ethnic disparities in infant mortality. Philadelphia has reported a big drop in infant deaths, from about 10 per 1,000 births in 2000, to about 8 in 2013. However, that rate is worse than those of Chicago, Los Angeles, and the Bronx, N.Y.

"Efforts to improve prenatal care, reduce teen pregnancies, decrease rates of cigarette smoking, … and increased breastfeeding are working together to reduce infant mortality," Kinsman said.

Jay S. Greenspan, chairman of pediatrics at Nemours/Sidney Kimmel Medical College at Jefferson University, cited other efforts that seem to be working, including campaigns to promote safe sleep practices, breastfeeding, and infant car seats. He said folic acid vitamin supplementation has contributed to the 11 percent reduction in the leading cause of infant death -- congenital malformations.

The third-leading cause of infant death, sudden infant death syndrome, or SIDS, fell by 29 percent, the steepest drop, the new report found. Experts said this may reflect both safe sleep practices -- babies should be put to sleep on their backs and not share parents' beds -- and more accurate reporting of accidental suffocation deaths of infants who sleep with parents.