Copyright © 2005 by the Johns Hopkins Bloomberg School of Public Health
LETTERS TO THE EDITOR |
THE AUTHORS REPLY
1 Department of Pediatrics, Faculty of Medicine, McGill University, Montreal, Quebec, Canada
2 Department of Epidemiology and Biostatistics, Faculty of Medicine, McGill University, Montreal, Quebec, Canada
3 Perinatal Epidemiology Research Unit, Department of Obstetrics and Gynecology and Pediatrics, Faculty of Medicine, Dalhousie University, Halifax, Nova Scotia, Canada
4 Section of Epidemiology and Biostatistics, Department of Obstetrics, Gynecology and Reproductive Services, Robert Wood Johnson Medical School, University of Medicine and Dentistry of New Jersey, New Brunswick, NJ
We thank Dr. Smith (1) for his comments on our paper (2), and we acknowledge his prior use of time-to-event analysis of perinatal outcomes. We have in fact cited his work previously in a more relevant context (3). The use of the time-to-event approach dates to Yudkin et al. (4) (at least implicitly) and has been used by several others as well in a variety of contexts. Our statement was directed at the numerous adherents of the conventional approach (e.g., Hartley et al. (5), Scher et al. (6), and Cheung (7)).
Dr. Smith (1) makes an important point regarding intrapartum stillbirth. We disagree, however, that the only relevant denominator for such deaths is all births at a given gestational age. All living fetuses at that gestational age are at risk of the onset of labor and hence of shoulder dystocia and intrapartum stillbirth. Furthermore, in line with our earlier argument, we believe that the etiologic overlap among all stillbirths (antepartum and intrapartum) and neonatal deaths is substantial, making fetuses at risk the appropriate denominator in this context and, by extension, the survival analytic approach an appropriate analysis (2). In fact, there is a strong case for using fetuses-at-risk as the denominator for serious neonatal morbidity as well (8, 9), since it is generally accepted that many adverse neonatal outcomes may also have a prenatal etiology.
We have previously attempted to address the issue of the crossover and gestational age (10, 11). In our work, we demonstrated that the crossover pattern persisted for gestational-age-specific neonatal mortality as well as birth-weight-specific mortality, and that in fact the difference between mortality in infants of mothers who smoke and those who do not cannot be explained by differences in gestational age. We believe that time-to-event analyses (fetuses at risk or fetuses and infants at risk) provide one elegant solution that is more coherent.
We remain pleased that the debate over the appropriate analytic strategies for perinatal and infant outcomes continues.
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REFERENCES |
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- Smith GCS. Re: "A proportional hazards model with time-dependent covariates and time-varying effects for analysis of fetal and infant death." (Letter). Am J Epidemiol 2005;161:000–00.
- Platt RW, Joseph KS, Ananth CV, et al. A proportional hazards model with time-dependent covariates and time-varying effects for analysis of fetal and infant death. Am J Epidemiol 2004;160:199–206.
[Abstract/Free Full Text] - Joseph KS, Demissie K, Platt RW, et al. A parsimonious explanation for intersecting perinatal mortality curves: understanding the effects of race and of maternal smoking. BMC Pregnancy Childbirth 2004;4:7.[CrossRef][Medline]
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- Cheung YB. On the definition of gestational-age-specific mortality. Am J Epidemiol 2004;160:207–10.
[Abstract/Free Full Text] - Dodd JM, Robinson JS, Crowther CA, et al. Stillbirth and neonatal outcomes in South Australia, 1991–2000. Am J Obstet Gynecol 2003;189:1731–6.[CrossRef][ISI][Medline]
- Joseph KS, Allen AC, Lutfi S, et al. Does the risk of cerebral palsy increase or decrease with increasing gestational age? BMC Pregnancy Childbirth 2003;3:8.[CrossRef][Medline]
- Platt RW, Yang H, Kramer MS, et al. Maturity, growth and infant mortality: a new approach. (Abstract). Paediatr Perinat Epidemiol 2001;14:A24.
- Platt RW, Ananth CV, Kramer MS. Analysis of neonatal mortality: is standardizing for relative birth weight biased? BMC Pregnancy Childbirth 2004;4:9.[CrossRef][Medline]
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