American Journal of Epidemiology Advance Access originally published online on August 27, 2008
American Journal of Epidemiology 2008 168(9):993-994; doi:10.1093/aje/kwn196
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McElrath et al. Respond to "Disaggregating Preterm Birth"
Correspondence to Dr. T. F. McElrath, Division of Maternal-Fetal Medicine, Brigham and Women’s Hospital, 75 Francis Street, Boston, MA 02115 (e-mail: tmcelrath{at}partners.org).
Received for publication May 22, 2008. Accepted for publication May 27, 2008.
We thank Dr. Savitz for his thoughtful comments (1) regarding our analysis (2). We are grateful that he put our paper in a broader context.
It is not evident in the epidemiologic literature, but there is a subtle disconnect between the in-hospital world of caring for women at risk for preterm delivery and the world of preterm birth as an epidemiologic phenomenon. In the former, women present with a variety of conditions that are not always immediately recognizable or classifiable into the traditional and narrow categories of the latter. The advanced dilation of the cervix without uterine contractions characteristic of cervical insufficiency has a much different clinical trajectory than does the presentation of the patient with painful regular contractions and cervical change. Yet, if both of these presentations ultimately lead to a preterm delivery, then they both might potentially be categorized under the same epidemiologic rubric of "spontaneous" delivery. Similarly, the delivery of a severely growth-restricted fetus for nonreassuring testing and the delivery of an otherwise appropriately grown baby whose mother has new hypertension and urinary protein loss would both be epidemiologically classified as "indicated" preterm births, although their clinical antecedents were very different.
Dr. Savitz makes the point that viewing all preterm births as one entity diminishes the opportunity to identify antecedents and modifiers of more homogeneous entities that comprise preterm birth. He labels the problem overascertainment bias. Our work was able to address this matter in two ways: First, we affirmed that there are indeed a variety of clinical presentations that contribute to preterm delivery and, second, found that these presentations are, in reality, different expressions of two pathways leading to preterm delivery. Dr. Savitz and others have previously suggested that this type of organization might be appropriate (3–5).
Our analysis presents evidence for grouping preterm labor, preterm premature membrane rupture, placental abruption, and cervical insufficiency into a category associated with intrauterine inflammation. Likewise, our analysis groups the presentations of preeclampsia and delivery for fetal indications/intrauterine growth restriction into a category associated with aberrant placentation. We hope that perinatal epidemiology might move toward this type of biology-based categorization rather than the more empirically descriptive labels of "spontaneous" and "indicated" preterm delivery.
An analytical framework was used in the initial planning of this study. Before considering factor analysis, we viewed our examination of placenta microbiology and morphology as the best available strategy to search for the biologic underpinnings of the 6 clinical presentations that lead to preterm delivery. This is an approach that we have used previously (6). Factor analysis, here, allowed us to see if demographic and pregnancy characteristics, as well as exposures during pregnancy, added to the discrimination provided by the placenta.
We, too, welcome the replication of our categorization system into a more near-term population. Our work centered on deliveries of less than 28 weeks of gestation and, as such, represents an extreme but thankfully infrequent form of preterm birth. Recent work has suggested that the near-term preterm birth experience has not been as well understood as had been believed previously (7, 8). We predict, however, that a biologic approach will help to classify disorders in the near-term population.
Controversy continues about whether preterm labor is like term labor only earlier, or whether what contributes to preterm labor is qualitatively different from the phenomena leading to term labor. Where near-term labor fits in remains unclear. We offer these comments about labor as an example of what might apply to some other complications of pregnancy that lead to preterm delivery. We need to learn how much gestational age at birth can function as an indicator of effect modification.
We welcome attempts to replicate our findings, as well as attempts to see how well a biologic approach will help to classify the disorders leading to delivery near term.
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ACKNOWLEDGMENTS |
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Author affiliations: Division of Maternal-Fetal Medicine, Brigham and Women's Hospital, Boston, Massachusetts (T. F. McElrath); Department of Pathology, Beth Israel Deaconess Medical Center, Boston, Massachusetts (J. L. Hecht); Division of Newborn Medicine, Tufts Floating Hospital for Children, Boston, Massachusetts (O. Dammann); University of North Carolina, Chapel Hill, North Carolina (K. Boggess); Channing Laboratory, Brigham and Women's Hospital, Boston, Massachusetts (A. Onderdonk); Department of Obstetrics and Gynecology, Bay State Medical Center, Springfield, Massachusetts (G. Markenson); Department of Maternal-Fetal Medicine, Wake Forest University Baptist Medical Center, Winston-Salem, North Carolina (M. Harper); Department of Obstetrics and Gynecology, University of Massachusetts Memorial Medical Center, Worcester, Massachusetts (E. Delpapa); and Neuroepidemiology Unit, Children's Hospital of Boston, Boston, Massachusetts (E. N. Allred, A. Leviton).
Conflict of interest: none declared.
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- Savitz DA. Invited commentary: disaggregating preterm birth to determine etiology. Am J Epidemiol (2008) 168(9):990–992.
[Abstract/Free Full Text] - McElrath TF, Hecht JL, Dammann O, et al. Pregnancy disorders that lead to delivery before the 28th week of gestation: an epidemiologic approach to classification. Am J Epidemiol (2008) 168(9):980–989.
[Abstract/Free Full Text] - Meis PJ, Ernest JM, Moore ML, et al. Regional program for prevention of premature birth in northwestern North Carolina. Am J Obstet Gynecol (1987) 157(3):550–556.[Web of Science][Medline]
- Savitz DA, Blackmore CA, Thorp JM. Epidemiologic characteristics of preterm delivery: etiologic heterogeneity. Am J Obstet Gynecol (1991) 164(2):467–471.[Web of Science][Medline]
- Klebanoff MA, Shiono PH. Top down, bottom up and inside out: reflections on preterm birth. Paediatr Perinat Epidemiol (1995) 9(2):125–129.[Web of Science][Medline]
- Hansen AR, Collins MH, Genest D, et al. Very low birthweight placenta: clustering of morphologic characteristics. Pediatr Dev Pathol (2000) 3(5):431–438.[CrossRef][Web of Science][Medline]
- Young PC, Glasgow TS, Li X, et al. Mortality of late-preterm (near-term) newborns in Utah. Pediatrics (2007) 119(3):e659–e665.
[Abstract/Free Full Text] - Escobar GJ, Clark RH, Greene JD. Short-term outcomes of infants born at 35 and 36 weeks gestation: we need to ask more questions. Semin Perinatol (2006) 30(1):28–33.[CrossRef][Web of Science][Medline]
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