OC099: Reference intervals for cervical length in twin pregnancies: Comparison of a transversal and longitudinal approach

Abstract

Introduction: Short cervical length is an important risk factor for preterm delivery. However, because the cervical length changes throughout pregnancy adequate risk estimation needs to take into account gestational age (GA) at which the measurement is taken. We aimed to model cervical changes throughout pregnancy in order to be able to use Z scores, avoiding the confounding effect of GA. Materials and Methods: Cervical length was prospectively collected in singleton pregnancies, as part of routine antenatal care over a 3-year period. Measurements were taken at GA ranging from 16 to 36 weeks’ and only one measurement per pregnancy was used. Because cervical length measurements are not normally distributed, we used a non-parametric approach (LMS method) to best describe the distribution of measurements with gestation. Results: We included 6615 cervical length measurements. The LMS method identified changes in cervical length measurements across gestational age. We computed new reference charts and provide L,M and S values that allow to compute Z scores at any GA from any cervical length measurement ‘‘y’’ based on Z score = ((y/M)L − 1)/L × S). Conclusions: Cervical length measurements do not follow a normal distribution throughout gestation and require an adequate model. The model we developed allows easy Z scores calculation, therefore avoiding the confounding effect of GA and allowing quality control.