Models for the prediction of successful induction of labor based on pre-induction sonographic measurement of cervical length.

Objective: To examine the effect of pre-induction cervical length, parity, gestational age at induction, maternal age and body mass index (BMI) on the possibility of successful delivery in women undergoing induction of labor.

Methods: In 822 singleton pregnancies, induction of labor was carried out at 35 to 42(+6) weeks of gestation. The cervical length was measured by transvaginal sonography before induction. The effect of cervical length, parity, gestational age, maternal age and BMI on the interval between induction and vaginal delivery within 24 hours was investigated using Cox's proportional hazard model. The likelihood of vaginal delivery within 24 hours and risk for cesarean section overall and for failure to progress was investigated using logistic regression analysis.

Results: Successful vaginal delivery within 24 hours of induction occurred in 530 (64.5%) of the 822 women. Cesarean sections were performed in 161 (19.6%) cases, 70 for fetal distress and 91 for failure to progress. Cox's proportional hazard model indicated that significant prediction of the induction-to-delivery interval was provided by the pre-induction cervical length (HR=0.89, 95% CI 0.88-0.90, p<0.0001), parity (HR=2.39, 95% CI 1.98-2.88, p<0.0001), gestational age (HR=1.13, 95% CI 1.07-1.2, p=or<0.0001) and birth weight percentile (HR=0.995, 95% CI 0.99-0.995, p=0.001), but not by maternal age or BMI. Logistic regression analysis indicated that significant prediction of the likelihood of vaginal delivery within 24 hours was provided by pre-induction cervical length (OR=0.86, 95% CI 0.84-0.88, p<0.0001), parity (OR=3.59, 95% CI 2.47-5.22, p<0.0001) and gestational age (OR=1.19, 95% CI 1.07-1.32, p=or<0.0001) but not by BMI or maternal age. The risk of cesarean section overall was significantly associated with all the variables under consideration, i.e., pre-induction cervical length (OR=1.09, 95% CI 1.06-1.11, p<0.0001), parity (OR=0.25, 95% CI 0.17-0.38, p<0.0001), BMI (OR=1.85, 95% CI 1.24-2.74, p=0.0024), gestational age (OR=0.88, 95% CI 0.78-0.98, p=0.0215) and maternal age (OR=1.04, 95% CI 1.01-1.07, p=0.0192). The risk of cesarean section for failure to progress was also significantly associated with pre-induction cervical length (OR=1.11, 95% CI 1.07-1.14, p<0.0001), parity (OR=0.26, 95% CI 0.15-0.43, p<0.0001), gestational age (OR=0.83, 95% CI 0.73-0.96, p=0.0097) and BMI (OR=2.07, 95% CI 1.27-3.37, p=0.0036).

Conclusion: In women undergoing induction of labor, pre-induction cervical length, parity, gestational age at induction, maternal age and BMI have a significant effect on the interval between induction and delivery within 24 hours, likelihood of vaginal delivery within 24 hours and the risk of cesarean section.