Ultrasound assessments
The ultrasound assessments were carried out by four operators (X.X.Z., H. K. B., P.Y.L. and H.Q.Y.), who have had at least three year’s obstetric ultrasound experience and were not involved in the clinical management of the recruited women. When the participants were in the semirecumbent position with an empty bladder, a two-dimensional transperineal ultrasonography scan was performed to assess the AoP, MLA and HPD using a GE Logiq V2 ultrasound system (GE Healthcare, USA) with a 2-5 MHz curvilinear transducer, as previously described21-23. The fetal occiput position was assessed transabdominally or transperineally 36, and the position was defined using the 12-hour clock system (see SUS1)37. The AoP is the angle between the long axis of the pubic bone and the leading part of the fetal skull in the midsagittal plane 21 (Figure 1a). The HPD is the shortest distance between the outer bony limit of the fetal skull and the perineum in the transverse plane 22(Figure 1b). The MLA is defined as the angle between the anteroposterior axis of the maternal pelvis and the midline of the fetal head 23(Figure 1c).
Digital examinations were performed by experienced obstetricians or midwives to assess cervical dilation, fetal head position and fetal head station before the ultrasonographic scans. The fetal head station was defined as the distance from the lowest part of the fetal head to the ischial spine (i.e., from -3 to +3, with 0 at the ischial spine). The interval between the paired examinations was no more than ten minutes, and the researchers were blinded to the findings of both examinations. The first intrapartum ultrasonographic examination was performed during the first stage of labor when the cervix was fully effaced and dilated by at least 3-4 cm but no more than 6 cm (latent phase) (T1), and intrapartum ultrasonographic scans were performed every three hours after that (T2, T3 and T4) until delivery.
The clinical team followed the standard local protocol of the research unit to manage labor progress. With a fully effaced cervix and cervical dilation of at least 3 cm, the women in labor were admitted to the predelivery room for continuous fetal heart monitoring. Labor augmentation with oxytocin was considered if there was suboptimal uterine power and unsatisfactory progress, as demonstrated by a partogram, and there was no evidence of obstructed labor. Failure to progress was defined as longer than 12 hours in the latent phase or cervix failed to dilate at a rate of 1cm per hour for at least 2 hours.
Maternal clinical data, such as age, gestational age at delivery, height, body mass index (BMI), induced or spontaneous labor, premature rupture of the membrane, the use of epidural anesthesia, labor augmentation with oxytocin, the final mode of delivery and the indication for operative delivery, were recorded. Data on the neonatal outcomes were also collected: sex, birth weight, Apgar scores after 1 and 5 minutes and the admission of the newborn to the neonatal unit. Gestational age was determined by the date of the first day of the last menstrual period and confirmed by the first-trimester ultrasound measurement of crown-rump length38. BMI was calculated according to the standard formula, and overweight was defined as BMI ≥ 25 kg/m2. Successful internal rotation was defined as the fetal head rotating to the occiput anterior (OA) position from a non-occiput anterior (non-OA) position (transverse or posterior) during the labor. Vaginal delivery in our unit included spontaneous and operative delivery using forceps.