Cervical sliding sign and cervical funneling in the third trimester as predictors of spontaneous preterm birth in singleton pregnancy.

Background: Spontaneous preterm birth (sPTB) is a major cause of neonatal morbidity and mortality, and accurate prediction remains challenging. Sonographic transvaginal cervical length (CL) and uterocervical angle (UCA) are commonly used as general screening methods for predicting preterm birth. However, these methods are limited by the variability of reference values. Recently, cervical sliding sign (CSS) has been used as a sonographic marker for sPTB. However, few studies on its use in this context have been conducted. Therefore, the aim of this study was to ascertain the relationship between CSS in the third trimester and the risk of sPTB and to determine the possible additional risk if funneling is also present.

Methods: This prospective cohort study included 168 singleton pregnancies in the third trimester (28^0-7-34^0-6 weeks) with no known risk factors for preterm birth who delivered vaginally between March 1, 2023 and October 30, 2023, in Baoan Women's and Children's Hospital. The CSS of the uterine cervix and cervical funneling were observed. CSS was defined as the sliding of the anterior lip of the cervix over the posterior lip via the application of gentle and continuous pressure on the cervix from the anterior fornix via the transvaginal ultrasound probe. Cervical funneling was defined as the ballooning of the membranes into a dilated internal os with a closed external os, with protrusion of at least 15% of the entire CL on sonography. sPTB was defined as the birth of the fetus before 37 weeks of gestation. Logistic regression was used to assess the contribution of CSS and cervical funneling to the risk of sPTB. Variables included sliding degrees and funnels.

Results: Twenty-nine (17.26%) of the singleton pregnancies were CSS, and the area under the curve (AUC) for predicting preterm birth by CSS was 0.799 [95% confidence interval (CI): 0.713-0.885]. CSS had a specificity of 94.1%, a positive predictive value of 72.4%, and a negative predictive value of 92.1% for the prediction of sPTB. Cervical funneling was observed for 28 (16.67%) of the singleton pregnancies. The AUC for predicting preterm birth by cervical funneling was 0.648 (95% CI: 0.558-0.738). The AUC obtained by the combination of CSS and cervical funneling was 0.801 (95% CI: 0.710-0.892). The AUC for CSS was greater than that for funneling (0.799 vs. 0.648; P=0.001), and the AUC for the combination of CSS with cervical funneling in predicting preterm labor was significantly greater than that for funneling alone (95% CI: 0.801-0.648; P<0.001). However, there was no statistically significant difference in the AUC between CSS and CSS combined with funneling (0.799 vs. 0.801; P=0.89). Logistic regression analysis demonstrated that funneling did not significantly contribute to CSS in the prediction of spontaneous delivery before 37 weeks (funneling: odds ratio =1.224, z =0.311, P=0.76; CSS: odds ratio =27.903, z =5.628, P<0.001). CSS increased the predictive accuracy for sPTB by 30.545-fold (z =6.564; P<0.001), and funneling increased the predictive accuracy for sPTB by 5.519-fold (z =3.778; P<0.001).

Conclusions: CSS is a reliable and superior marker for predicting sPTB in singleton pregnancies as compared to cervical funneling. Although cervical funneling may also have some predictive value, CSS is more effective as a diagnostic tool for sPTB.