Birth weight linked to infant mortality.

The Centers for Disease Control and Prevention compared birth data for the United States from 2001 to 2002. They reported an increase in infant mortality for the first time since 1958. This is noteworthy because historically, infant mortality in the United States has been on the decline.

The majority of infant deaths were attributed to low birth weight. The increase is seen most dramatically in extremely low birth weight babies (less than 750 grams or 1 lb., 10 oz.). The number of extremely low birth weight babies increased by almost 500 births from 2001 to 2002. Most of these babies born at this weight die within their first year.

Often times low birth weights are due to multiple births. About 3 percent of births in the United States were multiple births, yet they made up about 25 percent of the overall increase in infant mortality. However, most of the babies born at extremely low birth weights were single births. Statistics showed that there was an increase in infant mortality for singletons while there was a decrease in infant mortality for multiples.

The research examined other contributing factors such as race, ethnicity, the mother's age, length of pregnancy, and the extent of the mother's medical risk factors. Infant mortality increased for all race and ethnic groups, though it was statistically more significant for babies of African-American women. Interestingly, most of the increase was for women in their peak childbearing years (ages 20-34). Although there was a slight increase in rates of anemia, diabetes and chronic hypertension, these conditions remain relatively rare.

Other factors that could affect the statistics are: how the data is reported and the medical management of birth. For example, because of advances in medical technology, babies who would not have survived may now be born and survive, if only for awhile. Thus, what would have been reported as a fetal death might now be reported as an infant death.

How pregnancy is managed could also be a significant issue. In the past ten years, there have been improvements in ultrasound and fetal diagnosis and more intensive monitoring of at-risk pregnancies. This may have resulted in an increased likelihood that a Caesarean birth will take place and a low birth weight baby will be born. In fact, in 2002, 57 percent of very low birth weight infants were born by Caesarean, up 3 percent from 2001.

Judging from the data available for 2003, there does not seem to be a continuing trend for an increase in infant mortality. Nonetheless, birth weight does remain a very important factor for survival in newborns.

Detailed findings from this research are published in "Explaining the 2001-2002 Infant Mortality Increase: Data from &e Linked Birth/Infant Death Data Set," on the Centers for Disease Control and Prevention website at www.cdc.gov/nchs