Infant and child health

Abstract

The U.S. infant mortality rate, defined as the number of deaths before age one per 1000 live births, fell from 12.6 in 1980 to 6.9 in 2000, a decline of 45 percent. (1) Over this same period, the total age-adjusted death rate in the United States fell by only 16.4 percent. We can decompose this decline in infant mortality into two components: changes in the healthiness of newborns and changes in the survival rate of newborns conditional on a given level of health. One widely used measure of newborn health, the rate of low birth weight births, is defined as the percentage of live births of babies who weigh less than 2500 grams or 5.5 pounds. The rate of low birth weight in the United States has actually risen since 1980, from 6.8 to 7.6 percent. (2) A large portion of the increase is attributable to the rise in multiple births, which have grown from 2 to 3 percent of all live births over the same period. However, even if we adjust for multiple births, the underlying healthiness of newborns in the United States has remained largely unchanged since 1980. In short, the remarkable increase in the survival rate of infants has resulted almost exclusively from advances in the technology of newborn care. Why, therefore, has the underlying morbidity of newborns, as proxied by the rate of low birth weight births, remained so immovable? Even more baffling, why has there been so little change in the rate of low birth weight despite increases in the prenatal inputs that many contend should lower its incidence. For instance, the percentage of women who initiate prenatal care in the first trimester increased from 76.3 in 1980 to 83.7 in 2000. The percentage of women who smoke during pregnancy fell from 18.4 in 1990 to 11.4 in 2002, while the number of infants served by the Special Supplemental Nutrition Program for Women, Infants and Children (WIC) has almost doubled since 1988. (3) Recent research by my colleagues and me suggests that previous estimates of the efficacy of many inputs designed to improve newborn health is probably inflated by favorable selection. The women who initiate prenatal care early, or who participate in WIC, are likely to be more motivated, less stressed, and more risk averse than the women who start care late or who do not participate in WIC. Too often we lack empirical methods for overcoming the problems caused by selection. In addition, in vetting their results, economists often neglect the clinical literature. Consider studies of the effect of programs to enhance maternal nutrition on infant health. Economic theory is helpful in specifying the demand for nutrition, but the effect of nutrition on fetal growth is a physiological, not an economic, relationship. For example, the consensus in the literature has been that "WIC works." In a recent study, economists reported that prenatal WIC participation was associated with a 50 percent decline in very preterm births, infants born before 33 weeks gestation. (4) These results were consistent with a widely-cited study by economist Barbara Devaney and colleagues in which WIC was associated with a decline of between 2.2 and 6.2 percentage points in rate of preterm birth. (5) Nationally, 9.7 percent of single births--versus twins, triplets, and other multiple births--were preterm in 1989. These are remarkable improvements, but they are strongly at odds with the clinical literature. In randomized trial after trial, clinical researchers have been unable to find any intervention that prevents preterm birth. In a candid editorial in the New England Journal of Medicine, a leading investigator writes: (6) "Trials measuring the effect of interventions at eliminating a single risk factor are numerous; uterine contractions have been suppressed, cervixes have been sewn shut, microorganisms have been eliminated, and social support, better nutrition, and prenatal care have been provided. When these factors have been studied in isolation, not one has resulted in a decline in preterm birth" (p. …