The costs of low birth weight

Low birth weight (LBW) infants experience severe health and developmental difficulties that can impose large costs on society. However, estimates of the return to LBW-prevention from cross-sectional associations may be biased by omitted variables, such as genetic factors. To address this, we compare the hospital costs, health at birth, and infant mortality rates between heavier and lighter infants from all twin pairs born in the United States. We also examine the effect of maternal smoking during pregnancy-the leading risk factor for LBW in the United States-on health among singleton births after controlling for detailed background characteristics. Both analyses imply substantially smaller effects of LBW per se than previously thought, suggesting two possibilities: 1) existing estimates overstate the true costs and consequences of LBW by at least a factor of four and by as much as a factor of twenty; or 2) different LBW-preventing interventions have different health and cost consequences, implying that policy efforts that presume a single return to reducing LBW will be suboptimal.