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Preterm Birth: Causes, Consequences, and Prevention
(Alexander et al., 2003). Racial, ethnic, and gender differences in birth weight for gestational age become increasingly prominent as pregnancies approach term (Alexander et al., 1999). At 40 weeks of gestation, birth weight for African American infants tends to be lower than those for white, Hispanic, and Native American infants.
Furthermore, despite overall dramatic reductions over the decades, racial and ethnic disparities in neonatal and infant mortality rates in the United States persist (Alexander et al., 2003). Although mortality rates are higher for full-term African American infants than for full-term white infants, the smaller and more preterm the infant is, the more of a survival advantage that preterm African American infants have over preterm white and preterm Hispanic infants (Alexander et al., 1999, 2003; Allen et al., 2000; Demissie et al., 2001). As gestational age-specific neonatal mortality has decreased over the last several decades, this gap has narrowed but still exists for the more immature or smaller preterm infants (Allen et al., 2000; Hamvas et al., 1996, Appendix B). Borrowing the pharmacologists’ concept of the 50 percent lethal dose (LD50), which is the point at which 50 percent of a population dies and 50 percent survives, that 50 percent point decreased from 26.8 weeks gestation in 1975–1979 to 24.5 weeks in 1990–1994 for white infants born in South Carolina (Allen et al., 2000). For African American infants, the gestational age at which 50 percent survive decreased from 25.2 to 23.9 weeks. The gap in survival between white and African American infants decreased during those two time periods, from a difference of 1.6 weeks to 0.5 week of gestation, respectively.
Although racial disparities in preterm birth rates and mortality rates have been noted for many years, the reasons for these differences are complex and not well understood. Some have suggested that many of the obstetric and neonatal intensive care advances (e.g., surfactant and antenatal steroid use) disproportionately improved the rates of survival for white preterm infants (who had higher incidences of RDS within each gestational age category) (Hamvas et al., 1996). Gender disparities, with higher mortality and pulmonary morbidity rates in preterm male infants are presumed to be based on not yet elucidated biological mechanisms (Stevenson et al., 2005). Others are concerned that neonatal mortality rates are higher in hospitals that serve predominantly minority populations (Morales et al., 2005). The reported racial and ethnic differences in risk factors for and presentations of preterm birth suggest that the etiologies of preterm birth may play a role (Ananth et al., 2005; Reagan and Salsberry, 2005). From 1989 to 2000, the rate of preterm birth following preterm labor increased by 3 percent for whites but decreased by 27 percent for African Americans (Ananth et al., 2005). Medically indicated preterm birth increased for both groups, but at different rates: 55 and 32 percent for whites and African Americans, respectively. Preterm births following ruptured membranes de-