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Preterm Birth: Causes, Consequences, and Prevention
creased for both groups: 23 and 37 percent, respectively. Declines in neonatal mortality were associated with increases in medically indicated preterm births in white preterm infants but with declines in preterm birth following labor or ruptured membranes in black preterm infants. An important question is how racial disparities in preterm birth are influenced by the broad social context (see Chapter 4).
An important consideration for all of these issues is that the method used to estimate gestational age may play a complex (and interfering) role. Postnatal estimates of gestational age, especially the Ballard and Dubowitz gestational age assessments, tend to overestimate gestational age in preterm infants, but the magnitude of the overestimation varies with race and ethnicity. Even when a variety of maternal sociodemographic variables, pregnancy complications, and the type of delivery are controlled for, African American infants had significantly higher mean postnatal gestational age estimates for each gestational age interval (Alexander et al., 1992). The Ballard gestational age estimate was higher for African Americans for each gestational age interval, whether gestational age was determined by LMP or prenatal ultrasound (for a subset of infants for whom these data were available). In this study, the proportion of preterm births changed, depending on whether gestational age was estimated from LMP or Ballard score, and the amount of change varied by race. Since the Ballard estimates gestational age from physical and neurological infant characteristics, these and other similar data raise the question as to whether African-American infants mature more rapidly than white infants.
In response to missing data for gestational age as determined by LMP on many U.S. birth certificates, a location for insertion of the clinical estimate of the infant’s gestational age was added to U.S. birth certificates in 1989 and was intended to be a cross-check of the gestational age determined by LMP. It is likely that postnatal estimates factored into the clinical gestational age estimates recorded on the birth certificate, although the magnitude of this tendency is unknown. Any data obtained by using these clinical gestational age estimates are therefore difficult to interpret, especially when one is trying to discern true racial and ethnic differences from artifacts of data reporting (i.e., the use of postnatal gestational age estimates). This problem has implications for the use of the clinical gestational age estimates to evaluate differences in gestational age distributions, birth weights for gestational age, and gestational age-specific mortality data. Rather than reverting to a reliance only on birth weight data, this problem argues for moving toward the adoption of early ultrasound to establish or confirm the gestational age or EDC for all pregnancies. The earlier in gestation that the prenatal ultrasound is performed, the greater the validity that the gestational age estimates for all racial and ethnic groups becomes.