Subpubic arch angle and occult obstetric anal spincter injury in primiparous women: a prospective observational study

Occult obstetric anal sphincter injuries (OASI) are frequently underdiagnosed after vaginal delivery and may contribute to long-term pelvic floor dysfunction. Maternal pelvic anatomy has been suggested to influence the risk of sphincter injury; however, the role of the subpubic arch angle remains insufficiently explored. This prospective observational study included 300 primiparous women aged 18–40 years who underwent vaginal delivery with mediolateral episiotomy. The subpubic arch angle was measured antenatally using transperineal ultrasonography. All participants underwent ultrasound evaluation of the anal sphincter before delivery to exclude pre-existing injuries. External anal sphincter integrity was reassessed 24 h postpartum to identify occult injuries. Associations between the subpubic arch angle, maternal and neonatal characteristics, and the presence of occult anal sphincter injury were analyzed. Receiver operating characteristic (ROC) analysis was performed to explore an exploratory threshold of the subpubic arch angle for descriptive and comparative purposes. Occult external anal sphincter injury was identified in 96 women (32.0%). Women with sphincter injury had a significantly narrower mean subpubic arch angle compared with those without injury (109.24 ± 11.29° vs. 112.79 ± 11.39°, p = 0.012). Maternal height was also significantly lower in women with sphincter injury (p < 0.05). No significant associations were observed between occult sphincter injury and fetal biometric parameters, labor induction, or the duration of the second stage of labor. ROC analysis demonstrated limited discriminative ability of the subpubic arch angle (AUC = 0.587; 95% CI 0.518–0.656). Using an exploratory threshold of 111.55°, occult sphincter injury was more frequent among women with narrower subpubic arch angles (p = 0.003). In this prospective cohort of primiparous women, occult obstetric anal sphincter injuries were more closely associated with maternal and pelvic anatomical factors than with fetal characteristics. A narrower subpubic arch angle was associated with an increased risk of occult sphincter injury, although its standalone discriminative performance was limited. The subpubic arch angle may provide adjunctive anatomical information within a multifactorial risk assessment framework, rather than serving as an independent screening or diagnostic parameter. Larger prospective and multicenter studies are warranted to further clarify its clinical relevance.