Derivation of a time-to-delivery model using enrollment angle of progression in Korean nulliparous women with subsequent spontaneous vaginal delivery

Estimating the remaining time to delivery could aid intrapartum counseling. We derived a model relating the intrapartum angle of progression (AOP) to this interval in Korean nulliparous women in active labor. In this prospective study, 102 women with singleton, term, cephalic pregnancies were enrolled at the first examination documenting cervical dilatation ≥ 6 cm. Twenty-two underwent intrapartum cesarean delivery; the remaining 80, all of whom had spontaneous vaginal delivery, formed the derivation cohort. The predictor was enrollment AOP, and the outcome was log-transformed time to delivery (TTD). Higher enrollment AOP was associated with a shorter remaining time: β = − 0.134 per 1° (95% CI, − 0.166 to − 0.102); TTD = exp(21.04 − 0.134 × AOP); optimism-corrected R 2 = 0.44 (95% CI, 0.26–0.63); optimism-corrected calibration slope = 1.01 (95% CI, 1.00–1.05); and median absolute error = 36 min (95% CI, 26–43). Fetal station as recorded in routine practice explained essentially none of the variation in ln(TTD): apparent R 2 = 0.001 (95% CI, 0.000–0.049); adding enrollment AOP increased the apparent R 2 by 0.498 (95% CI, 0.364–0.641). The study was not registered. This single-center derivation model estimates TTD conditional on subsequent spontaneous vaginal delivery and requires external validation before clinical use.