Objective:To investigate the changes in cervical length (CL) after history- indicated cerclage through serial ultrasound measurements and the predictive value of short cervix (CL≤2.5 cm) before 24 gestational weeks after cerclage in preterm delivery before 34 weeks.Methods:In this retrospective study, clinical data of 145 singleton pregnancies with history-indicated McDonald cerclage at Peking University First Hospital from January 2010 to June 2021 were collected. CL was measured through transvaginal ultrasound and recorded during the perioperative period and at ≥20-24, ≥24-28, and ≥28-32 weeks of gestation. The participants were divided into ≥34 weeks group ( n=118) and <34 weeks group ( n=27) based on the gestational age at delivery. Chi-square or independent sample t test was applied for comparison between the two groups. The relationship between CL after cerclage and preterm birth before 34 week in the two groups was analyzed using multivariate logistic regression analysis. The risk of preterm birth in women with short cervix at 20-24 weeks of gestation after cerclage was estimated. The receiver operator characteristic curve was adopted for predicting the diagnostic value of postoperative CL on pregnancy outcomes. Results:(1) Among the 145 cases, 106 cases (73.1%) had term delivery, 27 cases (18.6%) had premature delivery before 34 weeks, among which seven cases (4.8%) had late miscarriage before 28 weeks (including two cases with survived viable infants). The overall live birth rate was 96.6% (140/145). (2) In the ≥34 weeks group, CL at ≥20-24, ≥24-28, and ≥28-32 weeks of gestation were significantly longer than those in the <34 weeks group [(3.35±0.73) vs (2.39±1.03) cm, t=5.69, P=0.008; (3.14±0.75) vs (2.14±1.10) cm, t=5.65, P=0.007; (2.91±0.85) vs (1.79±1.09) cm, t=5.84, P=0.005]. (3) Multivariate logistic regression analysis showed that CL at ≥20-24, ≥24-28, and ≥28-32 weeks after cerclage were the influencing factors of preterm birth before 34 weeks [ OR (95% CI)=0.885 (0.837-0.935), 0.886 (0.837-0.937), 0.890 (0.842-0.940), respectively, all P<0.001]. (4) The area under the curve of CL at ≥20-24, ≥24-28, and ≥28-32 weeks were 0.747, 0.734, and 0.799 for predicting delivery before 34 weeks, with the sensitivity of 60.0%, 60.0%, and 80.0%, and the specificity of 87.5%, 91.3%, and 76.9%, respectively. (5) At ≥20-24 weeks after cervical cerclage, women with a short cervix accounted for 17.2% (25/145), who were 12.7 times more likely to have premature birth before 34 weeks than those with a CL over 2.5 cm ( OR=12.7, 95% CI: 4.58-35.40, P<0.001). Conclusions:Among pregnant women with history-indicated cervical cerclage, CL after cerclage at ≥20-24, ≥24-28, and ≥28-32 weeks of gestation have predictive values for pregnancy outcomes. A short cervix at ≥20-24 weeks of pregnancy after cerclage could predict premature birth before 34 weeks.