A validated calculator to estimate risk of cesarean after an induction of labor with an unfavorable cervix

BACKGROUND: Induction of labor occurs in > 20% of pregnancies, which equates to approximately 1 million women undergoing an induction in the United States annually. Regardless of how common inductions are, our ability to predict induction success is limited. Although multiple risk factors for a failed induction have been identified, risk factors alone are not enough to quantify an actual risk of cesarean for an individual woman undergoing a cesarean. OBJECTIVE: The objective of this study was to derive and validate a prediction model for cesarean after induction with an unfavorable cervix and to create a Web-based calculator to assist in patient counseling. STUDY DESIGN: Derivation and validation of a prediction model for cesarean delivery after induction was performed as part of a planned secondary analysis of a large randomized trial. A predictive model for cesarean delivery was derived using multivariable logistic regression from a large randomized trial on induction methods (n= 491) that took place from 2013 through 2015 at an academic institution. Full-term (>= 37 weeks) women carrying a singleton gestation with intact membranes and an unfavorable cervix (Bishop score <= 6 and dilation >= 2 cm) undergoing an induction were included in this trial. Both nulliparous and multiparous women were included. Women with a prior cesarean were excluded. Refinement of the prediction model was performed using an observational cohort of women from the same institution who underwent an induction (n= 364) during the trial period. An external validation was performed utilizing a publicly available database (Consortium for Safe Labor) that includes information for > 200,000 deliveries from 19 hospitals across the United States from 2002 through 2008. After applying the same inclusion and exclusion criteria utilized in the derivation cohort, a total of 8466 women remained for analysis. The discriminative power of each model was assessed using a bootstrap, bias-corrected area under the curve. RESULTS: The cesarean delivery rates in the derivation and external validation groups were: 27.7% (n= 136/491) and 26.4% (n= 2235/8466). In multivariable modeling, nulliparity, gestation age >= 40 weeks, body mass index at delivery, modified Bishop score, and height were significantly associated with cesarean. A nomogram and calculator were created and found to have an area under the curve in the external validation cohort of 0.73 (95% confidence interval, 0.72-0.74). CONCLUSION: A nomogram and user-friendly Web-based calculator that incorporates 5 variables known at the start of induction has been developed and validated. It can be found at: http://www.uphs.upenn.edu/obgyn/labor-induction-calculator/. This calculator can be used to augment patient counseling for women undergoing an induction with an unfavorable cervix.