AbstractAim:To investigate whether the cervical length measured by transvaginal ultrasound at 35 to 36 weeks of gestation is predictive of spontaneous early onset of labor in pregnancies scheduled for elective cesarean section (CS).Study design:This was a prospective observational study on 167 women scheduled for elective CS at 39 weeks of gestation. The cervical length was measured ultrasonographycally at 35 to 36 weeks of gestation and the pregnancies characteristics were recorded. The presence or absence of spontaneous onset of labor before the time scheduled for elective CS was related to cervical length and pregnancies characteristics.Results:Of the 167 pregnancies enrolled, 25 (14.97%) underwent early spontaneous onset of labor before the time scheduled for the elective CS. The cervical length was significantly shorter in these women when compared with those delivering with an elective CS (21 vs. 30 mm z=5.49 P<0.0001). The presence of ≥2 previous CS as an indicator for elective CS resulted a significant predictor of early onset of labor in univariate analysis (P=0.01). Multivariate logistic regression analysis showed that cervical length (adjusted odds ratio 1.40; 95% confidence interval (CI) 1.22–1.61; P≤0.0001) was the only predictor for the early onset of labor. The area under the receiver–operating characteristics curve for the prediction of early onset of labor was 0.844 (95% CI 0.738–0.950) for cervical length as test variable. Kaplan-Meier analysis demonstrated that a cervical length <24 mm was significantly associated with a short time interval to delivery (log rank test χConclusion:Cervical length at 35–36 weeks of gestation provides information about the likelihood of spontaneous early onset of labor in pregnancies scheduled for CS and may be useful in individualizing the gestational age for elective CS.