In this prospective study, we presented a personalized risk assessment of the likelihood of a UOD in nulliparous term patients based on a combination of maternal and fetal sonographic measurements obtained before the onset of labor. According to our findings, a UOD can be mainly predicted by AOP, PAA, maternal age, maternal height, and EFW.
UOD is associated with a higher likelihood of adverse maternal and neonatal outcomes. Cesarean delivery performed during the second stage of labor is associated with a higher risk for complications, include postpartum hemorrhage, infection, and damage to the cervix. Operative vaginal deliveries are associated with a greater incidence of maternal anal sphincter injury, neonatal intracranial hemorrhage, subgaleal hematoma, and shoulder dystocia. Prelabor identification of patients at high risk for UOD is of great importance as it may assist in deciding the optimal mode of delivery for the specific patient.
It seems reasonable that in the complex labor process, which involves many confounding factors, using a combination of parameters to predict UOD would perform better than a single parameter would.
For many years, a physical examination has been the primary tool for assessing the maternal pelvis. However, clinical pelvimetry is in danger of becoming a lost art without appropriate training, and its application in the diagnosis of cases prone to protracted labor is limited [35–36].
Extensive research efforts have been invested in finding a non-invasive, objective, quantitative tool to predict a successful vaginal delivery. Various sono-pelvimetric parameters were reported to predict dysfunctional labor and unplanned operative delivery [12–22]. The trans-perineal sonographic measurement of the AOP, which describes the fetal head station, was proven to be a reliable parameter with high measurement reproducibility [24–30]. Among classic pelvimetric parameters, the infra-pubic angle, the primary measure of the pelvic outlet, is measured easily by ultrasound and was reported by our group and others [25–28] to be an independent risk factor for UOD.
Interestingly, according to our data, the difference in AOP was significant only when we compared the vaginal delivery (spontaneous or instrumental) and cesarean delivery groups. We did not find a difference between spontaneous vaginal delivery and any UOD (instrumental and cesarean delivery) group. These findings are consistent with findings in previous studies that a narrower AOP correlates with cesarean deliveries, while a wider AOP correlates with a spontaneous or instrumental vaginal birth [18–20].
Giuseppe et al.  explored the performance of a predictive model for detecting the need for UOD and reported that HC and pubic angle were independent risk factors for UOD, and the combination of maternal height, pubic angle, and head circumference showed the best results. Others have reported similar results regarding the effect of a large head circumference and an unplanned cesarean section [3–7].
However, in our study group, neither BPD nor head circumference negatively influenced the statistical model for predicting a UOD. According to the results reported by Mujugira et al. , maternal age modified the association between fetal head circumference and primary cesarean section. This association may reflect changes in pelvic cartilage, ligaments, collagen configuration, or other metabolic changes in the skeletal system secondary to aging. Indeed, in a study assessing the effect of PAA on the second stage of labor, our group  found an inverse relationship between PAA and maternal age.
Limitations of this study are a relatively high rate of UOD and lack of stratification of the analysis according to cervical status and Bishop score.
The study's main strengths rely on its prospective design, representing a low-risk population for obstetrical complications regarding maternal age, maternal BMI, fetal head indices, and birth weight; and in the fact that the patients and the attending obstetricians were blinded to the sonographic measurements.