More Babies Born at Very Low Birthweight

Linked to Rise in Infant Mortality in 2002

For Release: Monday, January 24, 2005

Contact: NCHS Press Office (301) 458-4800

Explaining the 2001-02 Infant Mortality Increase: Data From the Linked Birth/Infant Death Data Set. NVSR Volume 53, Number 12. 23 pp. (PHS) 2005-1120. [PDF – 1.2 MB]

An increase in the birth of very small infants is the major reason behind the increase in U.S. infant mortality in 2002, according to a report released today by the Centers for Disease Control and Prevention (CDC). The increase in infant mortality, from 6.8 infant deaths per 1,000 live births in 2001 to 7.0 in 2002, was the first increase in the infant mortality rate since 1958 and was reported last year by CDC.

Overall, there were 27,970 infant deaths in 2002 compared with 27,523 infant deaths in 2001. Provisional data for 2003 suggest that the increase noted in 2002 may not be continuing.

The number of extremely small babies (weighing less than 1 lb, 10.5 oz or 750 grams at birth) increased by almost 500 births from 2001 to 2002. The increase occurred primarily among mothers in the peak childbearing ages of 20–34 years and occurred across most racial and ethnic groups. While infant mortality rates had declined for these vulnerable small babies, the majority of babies born at this weight still die within the first year of life. Multiple births may also contribute to the increase in low birthweight infants. 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 rise was due to an increase for babies born in single deliveries.

The data from CDC’s 2002 Linked Birth/Infant Death Data Set enables researchers to examine factors, in addition to birthweight, that may have contributed to infant mortality. The report looks at patterns by race or ethnicity, mother’s age, length of pregnancy, multiple versus single birth, and other factors.

Researchers at CDC’s National Center for Health Statistics also evaluated changes in the reporting of births, infant deaths, and fetal deaths; the extent of mother’s medical risk factors; and aspects of medical practice that are documented on the birth certificate as possibly contributing to the increase in the very low birthweight infants. Each of these factors, as well as others, may also have contributed to the increase. For example, the report documents a slight increase between 2001 and 2002 in rates of maternal anemia, diabetes, and chronic high blood pressure; however, these conditions remain relatively rare.

Also potentially important are recent changes in the medical management of pregnancy. In 2002, 57 percent of very low birthweight infants were delivered by cesarean, up 3 percent from 2001. Substantial changes in technology have occurred over the past decade, including improvements in fetal imaging and diagnosis. More intensive monitoring of at-risk pregnancies may have resulted in an increased likelihood that a cesarean delivery will take place and a low birthweight baby will be born.

Detailed findings from this research are published in “Explaining the 2001–2002 Infant Mortality Increase: Data from the Linked Birth/Infant Death Data Set” and available on the CDC/NCHS Web site.