A narrow subpubic arch angle is associated with a higher risk of persistent posterior occiput position at birth

OBJECTIVE:The aim of the present study was to determine whether the width of the subpubic arch angle (SPA) measured by 3D ultrasound is associated with the fetal occiput position at birth and the type of delivery MATERIAL AND METHODS: a series of nulliparous women with uncomplicated pregnancy were recruited at ≥37 weeks from two tertiary level centers. The SPA was measured by 3D transperineal US using the previously validated OVIX technique. Labor outcome was prospectively investigated in all cases. RESULTS: Overall, 368 women were included in the study group. Among these, fetal position at delivery was occiput anterior (OA) in 339 (91.6%%) and occiput posterior (OP) in 29 (7.8%). In the OP group compared with the OA a narrower SPA (104.3±16.8 vs 116.4±11.9 p≤0.0001) was found. Moreover, the SPA appeared significantly narrower in the women submitted to obstetric intervention compared with those undergoing spontaneous vaginal delivery. At multivariable logistic regression, SPA width and maternal height appeared significant predictors of both the fetal occiput position at birth and the risk of operative delivery. The best cut-off value of SPA for predicting occiput posterior at birth was 90.5°. CONCLUSIONS: A narrow subpubic arch angle is associated with a higher risk of persistent posterior occiput position at birth and of operative delivery.