Cervical length following cerclage as a predictor of spontaneous preterm birth

Objective: We aimed to evaluate the association between serial transvaginal cervical length measurements following McDonald cerclage and spontaneous preterm birth (PTB). Methods: This retrospective study included singleton pregnancies with cerclage performed during 2010–2024. Cerclage was placed prophylactically (n = 109) based on obstetric history, or emergently (n = 46) due to ultrasound findings. Cervical length was measured by transvaginal ultrasound before and after cerclage, and at 2-week intervals until 32 weeks. Results: For the prophylactic group, the median cervical length was shorter among those who delivered PTB < 37 weeks (n = 23) than term: at 21–22 + 6 weeks (2.5 vs. 3.9 cm, p = 0.042), 23–24 + 6 weeks (2.0 vs. 3.4 cm, p = 0.016), 25–26 + 6 weeks (3.0 vs. 3.8 cm, p = 0.042), and 31–32 + 6 weeks (2.4 vs. 3.4 cm, p = 0.015). In multivariable analysis adjusted for history of PTB, progesterone use, and gestational age at cerclage placement, shorter cervical length, at 23–24 + 6 weeks (adjusted odds ratio [aOR] 4.13, 95% confidence interval [CI] 1.23–13.89, p = 0.021) and at 25–26 + 6 weeks (aOR 3.39, 95% CI 1.08–10.64, p = 0.037), was independently associated with PTB < 37 weeks. Cervical length at 25–26 + 6 weeks was associated with PTB < 32 weeks (aOR 4.76, 95% CI 1.20–19.60, p = 0.027). For the emergency group, the median cervical length was shorter among those who delivered < 32 weeks than later, at 23–24 + 6 weeks (1.4 vs. 3.2 cm, p = 0.049) and 25–26 weeks (1.5 vs. 2.3 cm, p = 0.041). Conclusion: Serial cervical length monitoring after cerclage provides a clinically relevant prediction of spontaneous PTB.