The role of intrapartum ultrasonography performed at the onset of the active phase of labor in predicting the time of delivery in nulliparous women

Objective: The aim of this study is to evaluate the predictive value of intrapartum ultrasound parameters measured at the onset of the active phase of labor (6 cm cervical dilatation) in nulliparous women, including the angle of progression (AoP), head–perineum distance (HPD), occiput position, head direction, and midline angle (MLA), on labor duration. Materials and Methods: Between March 2024 and August 2025, a total of 57 nulliparous women at the onset of the active phase of labor (6 cm cervical dilatation) were prospectively evaluated. Labor duration, delivery mode, and neonatal outcomes were recorded. Statistical analyses included Spearman's correlation, multivariable linear regression, and receiver operating characteristic (ROC) curve analyses. Results: A significant negative correlation was found between AoP and labor duration (r = –0.394; p = 0.002), and ROC analysis demonstrated that AoP had a significant discriminative power in predicting delivery. For predicting birth within two hours, the AUC of AoP was 0.692 (95% CI: 0.548–0.836; p = 0.009), indicating a moderate discriminative ability. With a cutoff value of ≥124.5°, sensitivity was 62.5%, positive predictive value was 65.2%, and the likelihood ratio was approximately 2.6. For predicting delivery within four hours, the AUC was 0.761 (95% CI: 0.616–0.905; p < 0.001), indicating good discriminative ability. At this interval, the cutoff value of ≥119.5° yielded a sensitivity of 65% and a positive predictive value of 93.8%. A significant positive correlation was also observed between HPD and labor duration. In the ROC analysis for predicting delivery within the first hour, the AUC of HPD was 0.697 (95% CI: 0.549–0.844; p = 0.009), reflecting moderate discriminative ability. With a cutoff value of ≤39.5 mm, the negative likelihood ratio was 0, and no patient with HPD above this threshold delivered within the first hour. This finding suggests that HPD is clinically useful in early identification of cases unlikely to deliver within the first hour. Head station was identified as the strongest independent predictor of labor duration (β = 0.258; p = 0.017). The difference in head station among deliveries within the first two hours was highly significant (p < 0.001), with a substantially higher likelihood of delivery within two hours observed in cases with head station ≥+1. MLA showed a weak and statistically non-significant correlation with labor duration (r = 0.055; p = 0.711). The mean labor duration in occiput anterior cases was shorter compared to occiput posterior and occiput transverse cases. Fetal head direction significantly influenced labor duration (H(2) = 9.20; p = 0.010), with significantly shorter durations in upward-directed heads. Additionally, prolonged labor was associated with significantly lower APGAR scores at both 1 minute (r = –0.324; p = 0.014) and 5 minutes (r = –0.305; p = 0.021). Conclusion: In nulliparous women, intrapartum ultrasound parameters measured at the onset of the active phase of the first stage of labor, including the angle of progression (AoP), head–perineum distance (HPD), head station, head direction, and occiput position, are clinically valuable predictors of labor duration. Keywords: Intrapartum ultrasonography; nulliparity; prospective study; angle of progression; head–perineum distance; labor duration; ROC analysis.