Infant mortality in the U.S. has declined 12 percent since 2005 after holding steady for many years, according to data released Wednesday by the Centers for Disease Control and Prevention.

The infant mortality rate in 2011 was 6.05 deaths per every 1,000 live births, down from 6.87 in 2005, according to the report from the National Center for Health Statistics.

Some of the biggest gains were seen in Southern states, though the region still has the highest infant mortality rates overall. The highest rates are in Mississippi and Alabama.

Infant mortality also declined for all major racial and ethnic groups, but dropped the most among African-Americans, who continue to have double the infant death rate of whites.

The gains for the Southern states and for black women are positive, but the gaps are still considered a major problem, said study leader Marian MacDorman, a senior statistician at the health statistics agency.

The improvements in infant mortality rates are due to declines in the leading causes of death for babies before they reach their first birthday: low birth weight, congenital malformations, sudden infant death syndrome and maternal complications, according to the report.

MacDorman also cited improved medical care and increased awareness of the possible dangers of elective deliveries and preterm birth.

"These recent efforts to limit elective deliveries are beginning to change the culture around early delivery," she said. "That is very positive."

Efforts around the country to reduce infant mortality are finally having an impact, said Michael Fraser, chief executive of the Association of Maternal and Child Health Programs in Washington. Now the goal will be to maintain those efforts, he said.

Health officials have started recognizing that good medical care during pregnancy cannot undo a lifetime of exposure to stress and bad environments.