Researchers considered a baby's death preterm-related if the cause occurred in more than 75% of infants born before 37 weeks gestation and the death was a direct result of being born too early.
The report by the CDC's division of vital statistics links infant death records to birth information. The most recent statistics revealed few fluctuations in the overall U.S. infant mortality rate from 2004 to 2005 (6.78 deaths per 1,000 live births vs. 6.86 deaths per 1,000 live births.)
Overall death rates were lowest for babies born to mothers of Central and South American (4.68 per 1,000) and Asian or Pacific Islander origin (4.89 per 1,000) and highest for non-Hispanic black mothers (13.63 per 1,000.) Among Hispanics, infant mortality rates ranged from 4.42 for mothers of Cuban descent to 8.30 for women of Puerto Rican origin.
However, preterm-related infant deaths jumped from 2000 to 2005 (34.6% to 36.5%).
Babies born to non-Hispanic black women were 3.4 times more likely to die from preterm-related causes than were non-Hispanic white women. The preterm-related infant mortality rate for Puerto Rican mothers was 87% higher than that seen in non-Hispanic white moms.
Doctors have seen a steady increase in the number of preterm and low-birth-weight births since the mid-1980s, according to background information in the report. Some of this is attributed to a rise in multiple births, in part due to more frequent use of fertility treatments, and to an increase in the number of women having C-sections and labor induced before the baby is full term.
In 2005, low birth weight or preterm babies were born more often to non-Hispanic black mothers than women of other origin. Mothers of Mexican origin had the lowest rates of low-birth-weight babies, and Asian or Pacific Islander mothers had the lowest rates of preterm births (10.7%).
"The differences in low birth weight and preterm births are major factors in the differences in infant mortality rates by race and ethnicity," the CDC researchers write in their report.
More than half of all infant deaths in the U.S. in 2005 occurred in babies born extremely early -- before 32 weeks gestation. Babies born slightly premature (34-36 weeks gestation) had three times the infant death rate than infants born 37 weeks or later. The leading causes of infant death were birth defects, low birth weight, and sudden infant death syndrome (SIDS).
Other factors linked to higher infant death rates included multiple deliveries (twins, triplets, or more), and mothers who were unmarried or born in the 50 states and Washington, D.C. Infant death rates were higher among boys than girls.
By Kelli Stacy
Reviewed by Louise Chang
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