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. Infant mortality rates ranged from 4.52 per 1,000 live births for Central and South American mothers to 13.35 for non-Hispanic black mothers. Infant mortality rates were higher for those infants whose mothers were born in the 50 states or the District of Columbia, were unmarried, or were born in multiple deliveries. Infant mortality was also higher for male infants and infants born preterm or at low birthweight. The neonatal mortality rate was essentially unchanged in 2006 (4.46) from 2005 (4.54). The postneonatal mortality rate decreased 4 percent, from 2.32 in 2005 to 2.22 in 2006. Infants born at the lowest gestational ages and birthweights have a large impact on overall U.S. infant mortality. For example, more than half of all infant deaths in the United States in 2006 (54 percent) occurred to the 2 percent of infants born very preterm (less than 32 weeks of gestation). Still, infant mortality rates for late preterm infants (34-36 weeks of gestation) were three times those for term infants (37-41 weeks). The three leading causes of infant death--congenital malformations, low birthweight, and sudden infant death syndrome--taken together accounted for 46 percent of all infant deaths. The percentage of infant deaths that were "preterm-related" was 36.1 percent in 2006. The preterm-related infant mortality rate for non-Hispanic black mothers was 3.4 times higher and the rate for Puerto Rican mothers was 84 percent higher than for non-Hispanic white mothers.