Federal health officials have found that U. S. preterm births dropped for two years in a row, but they still remain high compared with other developed countries.

The U. S. infant mortality rate was 6.68 infant deaths per 1,000 live births in 2006, a 3 percent decline from 6.86 in 2005, a report by the National Center for Health Statistics, of the Centers for Disease Control and Prevention, said.

The report also said that infant mortality rates were higher for infants whose mothers were unmarried, were born in multiple deliveries, were male infants and infants born preterm or at low birth weight. The post-neonatal mortality rate decreased 4 percent, from 2.32 in 2005 to 2.22 in 2006.

More than half of all U. S. infant deaths in 2006 occurred to the infants born considered very preterm, less than 32 weeks of gestation.

The report further said that however, infant mortality rates for late preterm infants, 34 to 36 weeks of gestation were three times those for babies born to term -- 37 to 41 weeks.

The highest rate, 13.35 per 1,000 live births, was for infants of black mothers with the lowest rate of 4.52 per 1,000 live births for infants of Central and South American mothers.

It was also noted by the report that the infant mortality rates were higher in the South and Midwest and lower elsewhere. For 2004 to 2006, infant mortality rates ranged from 10.63 in Mississippi to 4.93 for Massachusetts. (With Inputs from Agencies)