Fetal head circumference and subpubic angle are independent risk factors for unplanned cesarean and operative delivery

Introduction The aim of this study was to ascertain whether combined ultrasound assessment of fetal head circumference (HC) and maternal subpubic angle (SPA) prior to the onset of labor may predict the likelihood of an unplanned operative delivery (UOD) in nulliparous women at term. Material and methods Prospective cohort study of singleton pregnancies in cephalic presentation. Pregnancies experiencing UOD secondary to fetal distress were excluded. HC was assessed transabdominally and SPA values were obtained from a reconstructed coronal plane on three‐dimensional (3D) ultrasound performed translabially at 36–38 weeks of gestation. Maternal characteristics, HC expressed as multiple of median, and SPA were compared according to the mode of delivery. Logistic regression and receiver operating characteristics curve analyses were used to analyze the data. Results 597 pregnancies were included in the study. Spontaneous vaginal delivery occurred in 70.2% of the cases and UOD was required in 29.8%. There was no difference in pregnancy characteristics and birthweight between women who had a spontaneous vaginal birth compared with UOD. The HC multiple of median was larger (1.00 ± 0.02 vs. 1.03 ± 0.02, p ≤ 0.0001), whereas SPA was narrower in the UOD group (124.02 ± 13.64 vs. 102.61 ± 16.13, p ≤ 0.0001). At logistic regression, SPA (OR 0.91, 95% CI 0.89–0.93), HC multiple of median (OR 1.13, 95% CI 1.09–1.17) and maternal height (OR 0.95, 95% CI 0.92–0.99) were independently associated with UOD. When combined, the diagnostic accuracy of a predictive model integrating HC, SPA and maternal height was highly predictive of UOD with an area under the curve of 0.904 (95% CI 0.88–0.93). Conclusions Ultrasound assessment of fetal HC and maternal SPA after 36 weeks of gestation can identify a subset of women at higher risk of UOD during labor, for whom early planned delivery might be beneficial.