Results
The score comprised seven variables significantly associated with failed mid-cavity VAD: head-perineum distance ≥51 mm (p<0.001), bulky caput succedaneum (p<0.001), maternal height ≤1.54 m (p<0.001), duration of expulsive efforts before vacuum <21 minutes or >30 minutes (p=0.0013), duration from five centimetres to complete cervical dilation ≥3 hours (p=0.0091), term ≥42 weeks of gestation (p=0.032), and occiput-posterior or occiput-transverse fetal head position (p=0.041). The model was reliable (Hosmer-Lemeshow test =8.5; p=0.39) and accurate (concordance index =0.74). The threshold for a transfer to the operating theatre was set at 16.3 %. Advanced preparation of caesarean section equipment and anticipated extension of epidural analgesia were significantly associated with a decrease in the delay between VAD failure and birth by caesarean section (p<0.001).