In this retrospective study based on cervical length (CL) measurements between 20 and 24 + 6 weeks, we examined the ability of CL to predict spontaneous preterm birth (SPTB) in 222 twin pregnancies using the receiver-operating curve (ROC) analysis and an a priori cut-off. CL predicted SPTB before 34 weeks. Using the ROC the selected cut-off was 37.5 mm. Positive predictive value (PPV) and negative predictive value (NPV) regarding SPTB before 34 weeks for 37.5 mm were 15.7% and 5.3% respectively. Using the 5th percentile, PPV and NPV regarding SPTB before 34 weeks for 24 mm were 41.7% and 91.4%, respectively. The 5th centile of CL measurements should be employed in clinical practice. CL measurement is an adequate screening tool for SPTB since it has a high NPV. Studies on CL measurement and SPTB should explain which methodology they adopted to obtain a cut-off value and the rationale of their choice.
Keywords: Cervical length; preterm birth; twin pregnancy.