Relative position of cerclage – an ultrasound marker for prediction of preterm birth

A transvaginal cerclage (TVC) identified high in the cervix is associated with prolongation of pregnancy. However, the relative position of the TVC (length above the cerclage/total cervical length) may be a better predictor of cerclage success as it is proportional of total cervical length. This study evaluates the relative position of TVC and its association with spontaneous preterm birth, comparing its predictive value against other post-cerclage ultrasound parameters. A retrospective cohort study was conducted of singleton pregnancies with TVC. Demographic and ultrasound data were collected. Differences in TVC characteristics between those who delivered preterm (< 37 weeks) and term, including relative position of cerclage, were assessed using Student’s t-test where data was continuous and chi-squared test where categorical. Receiver operator curves were generated to evaluate predictive performance (SPSS version 29.0). Among 53 women, 25% (n = 13) delivered preterm. The relative position of TVC was significantly lower in preterm cases (mean 0.41 versus 0.52, p = 0.024), while total cervical length showed no difference. Length above the TVC was shorter in spontaneous preterm births (mean 11 mm versus 16 mm; p = 0.008). In those who delivered preterm, findings of funnelling and amniotic fluid sludge were more likely to be present after cerclage (p = 0.07 and p = 0.02 respectively). ROC analysis demonstrated good predictive ability for relative position (AUC 0.74) and length above the TVC (AUC 0.73), with an optimal threshold of 0.425 for relative position. Cervical length and cerclage height were poor predictors (AUC 0.43 and 0.59 respectively). Ultrasound assessment after cerclage can identify women at higher risk of spontaneous preterm birth. Relative position and length above the TVC are good predictors, whereas cervical length and cerclage height are not. Incorporating relative position of TVC position into clinical risk stratification warrants further evaluation.