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.