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Objectives: A narrow subpubic arch angle (SPA) has been associated with a higher risk of operative delivery and prolonged labor. The aim of this study was to evaluate the correlation between SPA and labor outcome in a cohort of women delivering a large-for-gestational-age (LGA) fetus. Methods: An observational study involving two Italian tertiary centers (Parma and Rome) was carried out. Nulliparous women referred to the antepartum clinic between 35 and 39 weeks due to an increased risk of having an LGA fetus at birth were prospectively selected for the study purpose. Within the study cohort, SPA measurements were obtained by means of transperineal 3D ultrasound. Elective caesarean section and birth weight below 3,750 g represented exclusion criteria. In the final study group, SPA values were compared between the patients who underwent spontaneous vaginal delivery (SVD) and those who were submitted to unplanned obstetric intervention (UOI) due to prolonged or arrested labor (vacuum delivery or caesarean section). Results: Overall, 129 women were included, and the mean birthweight of the neonates was 4,066 ± 263.03 g. SVD occurred in 63 patients (48.8%), whereas UOI due to prolonged or arrested labor was performed in 66 (51.2%), including 21 cases of vacuum delivery and 45 caesarean sections. The SPA was significantly smaller among women who underwent UOI than in those who achieved SVD (107.9 ± 13.4 vs. 120.7 ± 9.4°, p < 0.001). Furthermore, SPA width was inversely correlated with labor length (p < 0.001). Multivariable logistic regression analysis showed that a smaller SPA (OR 1.091, 95% CI 1.051-1.134, p < 0.001) and an increased birthweight (OR 1.002, 95% CI 1.000-1.004, p = 0.037) were independent risk factors for operative delivery. Conclusion: SPA measurement before labor is helpful in predicting the risk of operative delivery due to prolonged or arrested labor among nulliparous women delivering LGA fetuses.

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