Re: "Preterm Delivery Rates in North Carolina: Are They Really Declining among Non-Hispanic African Americans?"

K. S. Joseph1 and Cande V. Ananth2

1 Perinatal Epidemiology Research Unit, Departments of Obstetrics and Gynaecology and Pediatrics, Dalhousie University, Halifax, Nova Scotia B3K 6R8, Canada
2 Divisions of Epidemiology and Biostatistics and Maternal-Fetal Medicine, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, New Brunswick, NJ 08901

Vahratian et al. (1Go) conclude that the recently observed decline in preterm birth among non-Hispanic African Americans in North Carolina is an artifact related to temporal reductions in the misclassification of full-term births (as preterm births). We are concerned that the bimodal birth weight distribution problem identified by Vahratian et al. (1Go) in 1989 was observed at 28–31 weeks' gestation but not at 32–36 weeks. This is important, because most of the decline in preterm birth among non-Hispanic African Americans in North Carolina and the United States (2Go–4Go) was observed at 32–36 weeks' gestation (from 14.4 percent in 1989 to 12.9 percent in 1999 (1Go)) rather than at 28–31 weeks' gestation (from 2.2 percent in 1989 to 1.9 percent in 1999 (1Go)).

More importantly, our recent studies of singletons (5Go) and twins (6Go) show that changes in preterm birth between 1989 and 2000 among Blacks in the United States have been primarily influenced by declines in spontaneous preterm birth and preterm birth following ruptured membranes. Over the same period, there has also been a concurrent, substantial increase in medically indicated preterm birth among Blacks (5Go, 6Go). These patterns provide at least a partial explanation for the rising rates of preterm birth among Black livebirths with birth weights less than 2,500 g (the other piece of evidence offered by Vahratian et al. (1Go)). Medically indicated preterm birth is associated with a much higher rate of growth restriction than spontaneous preterm birth and preterm birth following ruptured membranes (e.g., small-for-gestational-age rates of 22.3 percent vs. 8.0 percent and 8.6 percent, respectively (7Go)). Thus, the restriction to preterm births with birth weights less than 2,500 g would have largely resulted in a focus on trends in medically indicated preterm birth. It is also noteworthy that the restriction to preterm births of <2,500 g is excessively stringent (the 50th percentiles of birth weight for gestational age at 34, 35, and 36 weeks are 2,667 g, 2,831 g, and 2,974 g, respectively, according to the US reference standard for fetal growth (8Go)). The use of a more appropriate criterion for excluding misclassified full-term births—such as the exclusion of preterm livebirths with a birth weight higher than the 90th percentile of birth weight for gestational age (8Go)—would show that preterm birth among non-Hispanic African Americans in the United States declined from 1989 to 1999.

We agree with Vahratian et al. (1Go) that the quality of gestational age information has improved at 28–31 weeks' gestation. This proposition is also supported by the temporal decline in large-for-gestational-age livebirths at preterm gestation among both Whites and Blacks (9Go). Nevertheless, errors in gestational age at less than 32 weeks can only explain a small part of the recent changes in preterm birth and preterm birth subtypes among Blacks in the United States. The preponderance of the evidence suggests that there has been a real decline in overall preterm birth among non-Hispanic African Americans, and this finding deserves closer scrutiny.

References

  1. Vahratian A, Buekens P, Bennett TA, et al. Preterm delivery rates in North Carolina: are they really declining among non-Hispanic African Americans? Am J Epidemiol 2004;159:59–63.[Abstract/Free Full Text]
  2. Demissie K, Rhoads GG, Ananth CV, et al. Trends in preterm birth and neonatal mortality among blacks and whites in the United States from 1989 to 1997. Am J Epidemiol 2001;154:307–15.[Abstract/Free Full Text]
  3. State-specific changes in singleton preterm births among black and white women—United States, 1990 and 1997. MMWR Morb Mortal Wkly Rep 2000;49:837–40.[Medline]
  4. Joseph KS, Demissie K, Kramer MS. Obstetric intervention, stillbirth, and preterm birth. Semin Perinatol 2002;26:250–9.[ISI][Medline]
  5. Ananth CV, Joseph KS, Oyelese Y, et al. Trends in preterm birth subtypes among singletons in the United States, 1989 through 2000: impact on perinatal mortality. Obstet Gynecol (in press).
  6. Ananth C, Joseph KS, Demissie K, et al. Trends in twin preterm birth subtypes and impact on perinatal mortality: United States, 1989 through 2000. Am J Obstet Gynecol 2005;191(suppl):S26. (Abstract 58).
  7. Villar J, Abalos E, Carroli G, et al. World Health Organization Antenatal Care Trial Research Group. Heterogeneity of perinatal outcomes in the preterm delivery syndrome. Obstet Gynecol 2004;104:78–87.[CrossRef][ISI][Medline]
  8. Alexander GR, Himes JH, Kaufman RB, et al. A United States national reference for fetal growth. Obstet Gynecol 1996;87:163–8.[Abstract/Free Full Text]
  9. Ananth CV, Wen SW. Trends in fetal growth among singleton gestations in the United States and Canada, 1985 through 1998. Semin Perinatol 2002;26:260–7.[ISI][Medline]




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