Variables associated with successful cervical ripening (Bishop score > 6) were: gestational age, PROM, and Bishop score at admission. These results are consistent with those reported in the literature regarding the induction of labor process in general15. Obesity and estimated fetal weight (EFW), factors widely known as predictors of IoL failure8.16, are also risk factors for failure of cervical ripening. In contrast, estimated fetal weight (EFW) was not reported as a predictor of cervical under-ripening in studies conducted by Daykan6 and Hiersch5.
We found no association between successful cervical ripening and maternal age, parity, reason for induction, number of hours between rupture of membranes and insertion of vaginal dinoprostone, or neonatal factors.
We identified only two studies in the literature that presented models that predicted cervical ripening success with dinoprostone slow-release vaginal devices. Hiersch et al.5 presented an initial predictive model composed of parity, cervical dilatation at admission and gestational age, with a ROC-AUC of 0.79 (95% CI 0.74–0.84) and a second, more complex model composed of maternal age , BMI, parity, cervical dilatation, effacement, indication for induction, gestational age and newborn weight, with a ROC-AUC of 0.80 (95% CI 0.75–0.85). Melamed and others.7 identify maternal age > 30 years, nulliparity, BMI ≥ 25, cervical dilatation < 1 cm, изтриване ≤ 50% и гестационна възраст > 37 weeks as predictors of cervical ripening failure and created a logistic regression model that could predict ≈50% of all failed ripening cases (R2 = 0.47).
However, we agree with the conclusions of Melamed et al.7and caution should be exercised when interpreting the results reported in the literature. Most studies analyzed successful induction of labor, defined as vaginal delivery within 24 hours8,9,17as an end result, without distinguishing induction from the preceding process of cervical ripening, so it is not possible to properly assess the cervical response to the action of vaginal dinoprostone without this outcome being influenced by additional intrapartum factors.
Regarding the process of cervical ripening, we should clarify that there is no universally accepted threshold score for determining a favorable or unfavorable cervix that tells us how to start induction. High Bishop scores have traditionally been associated with higher rates of vaginal birth success18,19. However, there are studies that question the reliability of Bishop scores in predicting the final outcome of labor20. In our study, we consider the process of cervical ripening to be successful after obtaining a BS > 6 with vaginal dinoprostone administration, based on the results obtained in most randomized trials and in clinical guidelines for induction of labor21,22.
Regarding the selection of the most adequate prediction model, many studies have compared the predictive ability of cervical length (CL) measured by ultrasound versus the Bishop score for induction of labor outcomes, with conflicting results.23,24,25,26 . A systematic review by Cochrane in 201527found no significant differences between the two methods (CL vs. BS) in rates of vaginal births, caesarean sections, and NICU admissions and concluded that there was insufficient evidence to recommend the use of CL over standard digital examination in assessing maturation on the cervix.
In addition to cervical length (CL) measured by ultrasound, fetal fibronectin has also been investigated in the assessment of cervical ripening, but was not found to be superior to the Bishop score15. Given the evidence from the published data and the ease of its replication, Model C, in which the only variable added was the Bishop score, was our chosen model.
The main limitations of this study are related to the sample size, which is relatively small compared to other studies5.8. Also, our predictive model was created based on the sociodemographic and obstetric characteristics of a Caucasian/Hispanic population, so it needs to be externally validated in another type of population before being used.
Furthermore, this study only analyzed obstetric and sociodemographic characteristics traditionally associated with success in labor induction15,28. However, these data should be interpreted with caution, as there is still some lack of understanding of the physiological phenomena associated with the onset of labor and cervical ripening, and there are large biological differences between mothers in the normal birth process29.
As for strengths, we can mention its forward-looking observation design, which makes the collection of variables more comprehensive and complete. Moreover, all patients included in the study were treated based on a homogeneous protocol for indication of delivery with clear indications at the end of the same, reducing the possible bias associated with its use.