Objective:To explore the clinical significance of cervical length (CL) measured by transabdominal ultrasound during fetal structural anomalies screeing at 20-24 +6 weeks of gestation. Methods:This was a retrospective nested case-control study based on a prospective longitudinal cohort of "Prediction and Prevention of Early-onset Preeclampsia", which recruited 4 995 singleton pregnant women at the gestational age of 11-13 +6 weeks in Nanjing Drum Tower Hospital from April 2019 to August 2022. All the subjects underwent second-trimester ultrasound screening for fetal structural anomalies in our hospital with image records. This study excluded the women who were lost to follow-up, underwent cervical cerclage, terminated the pregnancy due to personal or social factors, or had miscarriage before 20 weeks of gestation, and those with iatrogenic preterm births, intrauterine fetal death or no second-trimester cervical sonography images. Propensity score matching was used to match pregnant women with spontaneous preterm birth ( n=101) and those with full-term delivery ( n=101) in a 1∶1 ratio, with factors of maternal age, body mass index, preterm birth history, cesarean section history, and pregnancy interval ≥5 years. CL was measured based on the retained ultrasound images. Nonparametric test or Chi-square test were used for statistical analysis. Receiver operating characteristic (ROC) curve was used to evaluate the correlation between CL measured by transabdominal ultrasound in the second trimester and spontaneous preterm birth. Results:The CL measured by transabdominal ultrasound at 20-24 +6 weeks of gestation was significantly shorter in the spontaneous preterm birth group than that in the full-term group [2.8 cm (2.5-3.3 cm) vs. 3.4 cm (3.0-3.9 cm), Z=－5.85, P<0.001]. If CL<3.4 cm was used as the cut-off value for predicting spontaneous preterm birth (20-36 +6 weeks), the specificity and the sensitivity were 0.50 and 0.77, respectively, and the sensitivity reached 0.92 for predicting preterm birth before 32 weeks and 1.00 for predicting preterm birth before 28 weeks. If CL<3.7 cm was used as the cut-off value, the specificity and the sensitivity were 0.36 and 0.87, respectively, and the sensitivity was 1.00 for predicting preterm birth before 32 weeks. The efficacy of preterm birth screening at 28-36 +6 weeks of gestation was comparable to that at 20-36 +6 weeks, if CL<3.4 cm and CL<3.7 cm were used as the cut-off value, the sensitivity were 0.76 and 0.86, respectively. Conclusion:Transabdominal ultrasound measurement of CL in the second trimester can be a preliminary screening to determine whether further transvaginal ultrasound measurement of CL is needed for women without a history of preterm birth or late spontaneous abortion.