Infant deaths increased in 2002 for the first time since 1958. And now CDC researchers know why: more births of very small babies.

However, there is preliminary evidence that this disturbing trend has not continued, the CDC report says.

“Birthweight is one of the most important predictors of an infant’s survival chances,” writes lead researcher Marian F. MacDorman, PhD, a CDC epidemiologist. “The infant mortality rate for very low birthweight infants [less than 1 lb, 10.5 oz] was 105 times higher than the rate for infants born weighing [5 lb, 8 oz] or more.”

The numbers:

Overall, there were 27,970 infant deaths in 2002 compared with 27,568 in 2001. In 2001, there were 6.8 deaths per 1,000 live births. In 2002, there were 7.0 infant deaths per 1,000 live births.

During that same time period, the numbers of very low birthweight babies (search) increased by almost 500 births. This increase occurred among mothers aged 20 to 34 and across most racial and ethnic groups, the report shows.

Though infant deaths had been declining for these vulnerable, small babies, the majority of babies born at this weight still die within the first year of life.

Among the reasons for these very small babies and the higher infant death rate:

Preterm Births: A 3 percent increase in infant deaths among preterm babies (less than 37 weeks of gestation). In 2002, the death rate for preterm infants was 15 times – and the rate for very preterm infants was 75 times – the rate of term infants. More than two-thirds of infant deaths involved the 12 percent of infants born preterm; 54 percent of deaths involved the very preterm infants.

Multiple Births: A 3 percent increase in multiple births accounted for 25 percent of low birth weight babies.

Technological advancements have also played a role, MacDorman notes.

The increased use of assisted reproductive therapies (search), like in-vitro fertilization (search), has been linked with an increase in multiple-birth pregnancies and low birth weight babies. Changes in labor and delivery technologies, and aggressive treatment of premature rupture of membranes, may also be boosting the numbers, MacDorman writes.

Also, advances in neonatal medicine have prompted doctors to alter their perception of a very small infant’s chances of survival. In fact, what might have been reported as a “fetal death” may now be reported as a “live birth,” writes MacDorman. “Because these very high risk infants are likely to succumb within the first few hours or days of life, any significant shift in reporting could result in an increase in the infant mortality rate.”

Slight increases in anemia (search), diabetes, and chronic high blood pressure among mothers could have added to infants’ risks. Also, more intensive monitoring of at-risk pregnancies may have led to more very low birth weight babies via cesarean delivery. Numbers of cesarean births increased by 3 percent, she writes.

“The prevention of preterm and low birthweight delivery, and especially very preterm and very low birthweight delivery, should be central to efforts to further lower the U.S. infant mortality rates,” writes MacDorman.

By Jeanie Lerche Davis, reviewed by Michael W. Smith, MD