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by D’Vera Cohn

(June 2007) Newborns who weigh less than 5.5 pounds are at increased risk of health and behavioral problems down the road, but is their low weight the true cause of their later difficulties?

The answer to that question is disputed, and important. Government and nonprofit programs spend millions of dollars addressing the problem of low birth weight, but some question whether they are aiming at the right target.

A symposium sponsored by the Johns Hopkins Population Center and the Population Reference Bureau last month marshaled expert speakers to address the topic. The symposium at the National Press Club—titled “How Important Are the Long-Term Consequences of Low Birth Weight?”—was the first of an annual policy-oriented series planned by the two population organizations.

In one corner was Dalton Conley, a New York University sociologist, who argued that low birth weight contributes to so many later health and behavior problems that policymakers need to address it directly. In the other was Marie McCormick, a professor of pediatrics at Harvard University, who stated that while low birth weight is associated with later problems, it is not clear that it causes them directly.

Evidence That Low Birth Weight Matters

Babies whose birth weight is low not only have lower odds of survival, but also face increased risk of heart disease and diabetes, stunted growth, low I.Q. and other problems, according to a variety of studies. Although recent advances in neonatal intensive care have kept more of these infants alive, efforts to prevent low birth weight have lagged.

Conley said his own research, using the Panel Study of Income Dynamics, indicates that low birth weight, defined as less than 2,500 grams, or 5.5 pounds, has “predictive power” to alter the chance that a newborn will graduate high school on time. Low birth weight also makes it more likely that a baby later will be held back in school, enrolled in special education or classified as learning disabled, he said. Conley’s research focused on subsequent life differences of identical twins with different birth weights.

His own research also found a link between low birth weight and a higher risk of infant mortality among blacks, Conley said. Other research has found low birth weight infants also have health problems such as weak immune systems and neurological problems.

The implications of this research, Conley said, are that policymakers need to focus on known ways to counter low birth weight. One surprisingly effective intervention, he suggested, could be improved dental care for expectant mothers, because recent research reveals a connection between gum disease and premature contractions.

His finding that low-birth-weight black infants have a higher risk of early death, Conley said, should serve to focus attention on racial disparities in health care. There is “powerful evidence,” he said, that the elevated infant mortality in black low-birth-weight newborns is driven by their treatment after birth.

Low Birth Weight Is an Incomplete Explanation

But McCormick said the picture is more complicated than Conley allows. She questioned whether the standard definition of low birth weight was the right one. She pointed out that it is based not on bad consequences that occur at 2,500 grams, but on the location of 2,500 grams in the distribution of all birth weights. Gestational age could be a better measure of fetal maturity, she said.

McCormick said she knew of no biological mechanism by which low birth weight would cause health and cognitive problems later on.

Low birth weight in and of itself may be a less important cause than other factors, such as a mother who smokes, exposure to toxins, or multiple babies crowding each other in utero, she said. And she challenged the existence of a consistent low birth weight syndrome, noting that low birth weight babies will thrive or languish depending on the quality of hospital care they receive in infancy.

“We’ve had 30 years of misdirected policy” because of the focus on low birth weight to the exclusion of other things, she said. “If we don’t understand the target, we don’t understand what the policies may do.”

As far as policy steps, McCormick urged more emphasis on interventions after birth that are shown to have results. But she said they need to be sustained to have benefit.

D’Vera Cohn is senior editor at the Population Reference Bureau.