Selection of the potential risk factors
Based on the literature and on our personal experience, we selected different factors that could increase the risk of failed VADs. From a clinical point of view, only factors known at the beginning of the procedure were interesting and were therefore retained to develop the score.
Potential maternal risk factors included pre-pregnancy body mass index (kg/m²), maternal height (m), number of previous vaginal deliveries, number of previous C-sections, fundal height at admission to the labour room (cm), and ratio of fundal height to maternal height (m/m). This ratio had already been described in the literature as a predictor of labour arrest but never as a predictor of OVD failure22.
Potential fetal risk factors were the presence of a caput succedaneum and fetal head position (occiput-anterior position/occiput-posterior position/occiput-transverse position) at the beginning of the VAD.
Potential risk factors corresponding to labour and delivery characteristics were number of weeks of gestation, onset of labour (spontaneous or induced), duration from a cervical dilation of five centimetres to complete cervical dilation (hours), duration at complete cervical dilation before the beginning of expulsing efforts (minutes), duration of expulsive efforts before the beginning of the VAD (minutes), indication of the VAD1 (fetal distress/dystocia/other indications), and fetal head station at the beginning of the VAD defined by the head – perineum distance.
Fetal macrosomia has been described as a potential risk factor for failed OVD. However, the neonatal weight is unknown at the beginning of VAD, and no ultrasound fetal weight estimation method is perfectly reliable. Therefore, we decided not to include the neonatal weight or fetal weight estimation as potential risk factors.