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. (1) 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. (1
) in 1989 was observed at 2831 weeks' gestation but not at 3236 weeks. This is important, because most of the decline in preterm birth among non-Hispanic African Americans in North Carolina and the United States (2
4
) was observed at 3236 weeks' gestation (from 14.4 percent in 1989 to 12.9 percent in 1999 (1
)) rather than at 2831 weeks' gestation (from 2.2 percent in 1989 to 1.9 percent in 1999 (1
)).
More importantly, our recent studies of singletons (5) and twins (6
) 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 (5
, 6
). 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. (1
)). 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 (7
)). 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 (8
)). The use of a more appropriate criterion for excluding misclassified full-term birthssuch as the exclusion of preterm livebirths with a birth weight higher than the 90th percentile of birth weight for gestational age (8
)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. (1) that the quality of gestational age information has improved at 2831 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 (9
). 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