Objective: To develop and externally validate a prediction model to calculate the likelihood of prolonged induction of labor (induction start to delivery time >36 hours). Design: A retrospective cohort study Setting: Academic centers in the United States Population: Nulliparous women with singleton pregnancies and vertex presentation at term who underwent induction of labor and had a vaginal delivery at a single academic center. Methods: Analyses were limited to women with an unfavorable cervix. A backward stepwise logistic regression analysis was used to identify the factors associated with prolonged induction of labor. The final model was validated using an external dataset of the Consortium on Safe Labor after applying the same inclusion and exclusion criteria. We developed a receiver observer characteristic curve with area under the curve (AUC). Main outcomes of measures: Prolonged induction of labor Results: Of 2,118 women, 364 (17%) had prolonged induction of labor. Factors associated with prolonged induction of labor included body mass index, hypertension, fetal conditions, and epidural. Factors including younger maternal age, prelabor rupture of membranes, and a more favorable simplified Bishop score were associated with a decreased likelihood of prolonged induction of labor. In the external validation cohort, 4,418 women were analyzed, of whom 188 (4%) had prolonged induction of labor. The AUC of the final model was 0.76 (95%CI 0.73-0.80) for the external validation cohort. The online calculator was created and is available at https://medstarapps.org/obstetricriskcalculator. Conclusion: Our externally validated model was efficient in predicting prolonged induction of labor with an unfavorable cervix.