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Infants of mothers who are not married were shown to be at higher risk for poor outcomes. In 2005, infants of married mothers had an infant mortality rate of 5.25 per 1,000 live births, 45 percent lower than the rate for infants of unmarried mothers (9.61). Marital status may be a marker for the presence or absence of social, emotional and financial resources. Also, the infant mortality for mothers born in the 50 states and the District of Columbia (7.26) was 43 percent higher than the rate for mothers born elsewhere (5.08). A variety of hypotheses may account for the lower infant rate, including possible differences in migration selectivity, social support, and risk behaviors. Also, women born outside the 50 states and the District of Columbia have been shown to have different characteristics than their U.S. born counterparts with regard to socioeconomic and educational status.

Approximately two-thirds of all infant deaths in 2005 (18,782 out of 28,384) occurred during the neonatal period (from birth through 27 days of age). The neonatal rate for infants of non-Hispanic black mothers (9.13) was more than twice those for AIAN (4.04), non-Hispanic white (3.71), API (3.37), Mexican (3.78), Central and South American (3.23), and Cuban mothers (3.05). The highest postneonatal mortality rates were for infants of non-Hispanic black (4.50) and AIAN (4.02) mothers, about twice the rate for non-Hispanic white mothers (2.05). Postneonatal mortality rates for Mexican (1.75), API (1.51), and Central and South American mothers (1.46) were lower than for non-Hispanic white mothers.

The three leading causes of infant death-congenital malformations (20 percent), low birth weight (17 percent), and sudden infant death syndrome (8 percent)-accounted for 44 percent of all infant deaths. The fourth and fifth leading causes were maternal complications (6 percent) and cord complications (4 percent). Together, the top five causes accounted for 54 percent of all infant deaths in the United States in 2005. The percentage of infant deaths that were ''preterm-related'' increased from 34.6 percent in 2000 to 36.5 percent in 2005.

There were also additional statistics concerning infant mortality risks among education levels, levels of prenatal care, and for infants born to mothers who smoked during pregnancy. The infant mortality rate for mothers who completed 16 or more years of school was 4.15 in 2005. The rate was 51 percent lower than the rate for mothers who completed less than 12 years of education. The rate for mothers with a college degree was 3.60, 63 percent lower than the rate for mothers with less than a high school diploma (9.84).

The timing and quality of prenatal care received by the mother during pregnancy can be important to the infant's health and survival. In 2005, the mortality rate for infants of mothers who began prenatal care after the first trimester of pregnancy or had no care at all was 8.69 deaths per 1,000 live births, 40 percent higher than the rate for infants whose care began in the first trimester (6.20).

The infant mortality rate for infants of mothers who smoked was 11.44, 74 percent higher than the rate of 6.58 for non-smokers. The use of tobacco during pregnancy causes the passage of substances such as nicotine, hydrogen cyanide, and carbon monoxide from the placenta into the fetal blood supply. These substances restrict the growing infant's access to oxygen and can lead to adverse pregnancy and birth outcomes, such as low birth weight, preterm delivery, intrauterine growth retardation and infant mortality. Maternal smoking has also been shown to increase the risk of respiratory infections and inhibit allergic immune responses in infants.