Reply

Abstract

We thank Dr Sisti for his interest in our article.1Moroz L.A. Brock C.O. Govindappagari S. Johnson D.L. Leopold B.H. Gyamfi-Bannerman C. Association between change in cervical length and spontaneous preterm birth in twin pregnancies.Am J Obstet Gynecol. 2017; 216: 159.e1-159.e7Abstract Full Text Full Text PDF Scopus (21) Google Scholar We are very much in agreement with caution to carefully consider how cervical length cut-offs are determined. While a receiver operating characteristic cut-off derived from statistical indices may seem like a precise way to determine the mathematical “truth,” it is difficult to establish clinical grounds for the choice of one statistical test over another (eg, area under the receiver operating characteristic curve, Youden index). As Dr Sisti points out, the prevalence of preterm birth affects the positive and negative predictive values associated with a test result. Picking a cut-off based on centiles therefore requires an understanding of the prevalence of disease to be meaningful. In our study, we elected to examine the association between rate of change in cervical length and the risk for spontaneous preterm birth in twin pregnancies. While several studies have now examined cervical length at various points in gestation, our study focused on determining: (1) if cervical change in the midtrimester in twin pregnancies is associated with the pathologic outcome of preterm birth: and (2) if there is a degree of shortening that could identify patients at risk for preterm birth. While the results of our analysis may be of some utility in interpreting the results of serial cervical length measurements obtained in twin pregnancies, we primarily intend for the results of this study to be hypothesis-generating. Adequate prospective studies have not been conducted to test the performance of cervical length, either a single cut-point or change measured over time, as a screening test for preterm birth in twin pregnancies. At present, the Society for Maternal-Fetal Medicine recommends that routine cervical length screening not be performed for women with multiple gestation (grade 2B). Besides the limited evidence we have regarding risk prediction for preterm birth, part of this recommendation likely stems from the fact that there is a paucity of evidence to support interventions that might be undertaken as a result of a positive screening test. We are in agreement that we should aspire to undertake large prospective studies, in the mold of the Framingham Heart Study, through which we can learn about major risk factors for preterm birth in multiple gestation. The primary goal of such a study should be to derive a targeted population for an intervention. Until such an intervention exists and we can fully understand the risk profile of the intervention, it will be difficult to determine what are acceptable false-positive and -negative rates for our screening tests, whether singular or part of a risk assessment algorithm. Cervical length cut-off in twin pregnancy and preterm labor risk assessment algorithm: call for actionAmerican Journal of Obstetrics & GynecologyVol. 217Issue 2PreviewI read with interest the study “Association between change in cervical length and spontaneous preterm birth in twin pregnancies” by Moroz et al1 in the last issue of the American Journal of Obstetrics and Gynecology. Full-Text PDF