Preterm birth is the leading cause of neonatal morbidity and mortality. Conventional two-dimensional (2D) ultrasound measurement of cervical length (CL) has limitations in predicting spontaneous preterm birth (SPTB), making it difficult to accurately identify some high-risk pregnant women at early stages. This study aims to investigate the value of three-dimensional (3D) ultrasound measurement of cervical volume (CV) in predicting SPTB, providing a new direction for SPTB prediction.

A nested case-control trial was conducted on the pregnant women undergoing prenatal examinations in our department from January 2024 to November 2025. Transvaginal 2D ultrasound measurement of CL and 3D ultrasound measurement of CV were performed at 12 to 24 weeks and 28 to 32 weeks of gestation, respectively. After applying our inclusion and exclusion criteria, 237 cases with complete follow-up and medical records were included as study subjects. According to gestational age at delivery, the participants were divided into an SPTB group (n=36) for deliveries at 28 to <37 weeks, and a full-term delivery (FTD) group (n=201) for deliveries at ≥37 weeks. Univariate analysis was initially performed to compare various indicators between the 2 groups. To ensure model stability, 1∶1 propensity score matching (PSM) was employed for age, gestational weight gain, gravidity, parity, history of preterm birth, history of cervical conization, history of full-term delivery, and premature rupture of membranes. Univariate and multivariate conditional logistic regression analyses were applied to the PSM-matched SPTB (n=27) and FTD (n=27) groups to assess the independent predictive value of CL and CV for SPTB. ROC curves were plotted to evaluate the predictive efficacy of CL and CV for SPTB, calculating AUC values, optimal cutoff values, sensitivities, and specificities. Pearson correlation analysis was conducted to assess the association between CL and CV.

① Intergroup analysis showed statistically significant differences between the SPTB and FTD groups in terms of age, gestational weight gain, gravidity, parity, history of cervical conization, history of fullterm delivery, and premature rupture of membranes (P<0.05). ② After PSM, no significant differences were found between the SPTB and FTD groups in these above factors. ③ At 12 to 24 weeks of gestation after matching, CL was slightly lower in the SPTB group than in the FTD group, but the difference was not statistically significant (31.90±5.96 mm vs 33.70±6.97 mm, P=0.314); the CV in the SPTB group was significantly smaller than that in the FTD group (28.45±5.67 cm³ vs 32.72±6.23 cm³, P=0.011). At 28 to 32 weeks of gestation, the SPTB group exhibited both a significantly shorter CL (23.69±9.47 mm vs 30.58±8.48 mm, P=0.007) and a significantly smaller CV (22.70±7.64 cm³ vs 28.91±8.09 cm³, P=0.005) compared to the FTD group. ④ In the univariate model at 12 to 24 weeks of gestation, CV was significantly associated with the risk of preterm birth (OR=0.768, 95%CI: 0.612 to 0.964, P=0.023). When both CL and CV were included in the multivariate model, CV remained statistically significant (OR=0.769, 95%CI: 0.611 to 0.967, P=0.025), with smaller CV indicating greater preterm birth risk. The AUC value of CV was 0.680 (95%CI: 0.537 to 0.824, P=0.023), with an optimal cutoff value of 32.47 cm³, a sensitivity of 81.5% and a specificity of 51.9%. In the univariate model at 28 to 32 weeks of gestation, both CL (OR=0.864, 95%CI: 0.771 to 0.968, P=0.011) and CV (OR=0.827, 95%CI: 0.688 to 0.994, P=0.043) were significantly associated with the risk of preterm birth. When both were included in the multivariate model, neither showed statistical significance (P>0.05). Pearson correlation analysis revealed a strong positive correlation between CL and CV (r=0.689, P<0.001). The AUC value of CL was 0.717 (95%CI: 0.580 to 0.855, P=0.006), with an optimal cutoff value of 31.5 mm, a sensitivity of 81.5% and a specificity of 63%. The AUC value for CV was 0.706 (95%CI: 0.567 to 0.846, P=0.009), with an optimal cutoff value of 26.7 cm³, a sensitivity of 77.8% and a specificity of 63%. ⑤ CL was significantly shorter at 28 to 32 weeks of gestation compared to that at 12 to 24 weeks (27.13±9.56 vs 32.80±6.49 mm, P<0.001), and CV was obviously smaller (25.80±8.40 vs 30.58±6.28 cm³, P<0.001).

3D ultrasound measurement of CV at 12 to 24 weeks of gestation may have value in early identification of high-risk pregnant women for preterm birth.