Study design
The
prospective, blinded observational
study was conducted between
April
2020 and April 2021 at Women Hospital, Zhejiang University, school of
medicine. Nulliparous women were eligible if they had a viable singleton
cephalic presentation pregnancy at full term (≥ 37 completed weeks) with
clinical suspicion of CPD (either short maternal stature, unengaged
fetal presentation or suspected macrosomia). Exclusion criteria were
congenital malformations, maternal contraindications to vaginal delivery
and contraindications to the use of MRI such as severe claustrophobia
and metal implants.
The enrolled women had spontaneous onset of labor or performed induction
of labor. Failure to labor progress was defined according to national
guidelines11.
Cesarean
delivery was recommended if failure to generate cervical change after at
least 24 hours of oxytocin administration during the latent phase or the
rate of cervical dilation <1cm in 4 hours when the cervix was
> 6cm
dilated11. The
women were also excluded if they required cesarean deliveries with other
indications, such as fetal distress, placental abruption and
preeclampsia. Managing practitioners were blinded to the results of MRI
findings. Vaginal delivery included spontaneous and assisted operative
deliveries using forceps.
The study was approved by the local ethics and research committees
(IRB-20200044-R on April 04, 2020). In addition, written consent was
obtained from all women who agreed to participate.
MRI
pelvimetry and fetalbiometry
MRI measurements were performed within fourteen days before labor. All
antenatal MRI images were obtained using a 1.5-T unit system (GE Signal
HDxt; GE Healthcare, USA) and an eight-element phased-array
body coil. The enrolled women were
placed in a supine position without sedation. After a localizing
gradient echo sequence, the imaging protocol consisted of axial
T1-weighted fast spin-echo and sagittal T1 fast spin-echo sequences
(repetition time 520 ms, echo time 7.8 ms, 5mm slice thickness, 0.5 mm
gap, a field of view 36-40 cm). The scan
extended
down to the level of the lower margin of the pubic symphysis. Parameters
were measured using the institution’s picture archiving and
communication system (Zhejiang Greenlander I.T. Co., Ltd., Hangzhou,
China).
On transverse sections, bilateral
femoral head distance,
interspinous
distance, intertuberous distance and
subpubic angle were visualized and
measured (Figure S1). The midsagittal section measurements included the
following parameters: obstetric conjugate, pelvic width, sacral outlet
diameter, outlet diameter of the pelvis, sacrum length
and pelvic inclination (Figure S2).
Fetal biometry, including fetal biparietal diameter, head circumstance
and abdominal circumstance, were obtained from the 4-mm acquisition. All
parameters were measured independently by two radiologists with at least
five years of experience in gynecological MRI for inter-observer and
intra-observer reliability.
Maternal characteristics including age, gestational age at delivery,
height, weight, weight gain during pregnancy, induced or spontaneous
labor and the final mode of 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 first day of the last menstrual
period and confirmed by the first-trimester ultrasound measurement of
crown-rump length12.
Body mass index (BMI) was calculated
according to the standard formula.
Statistical analysis
The baseline characteristics were described using means (standard
deviations [SDs]) for continuous variables and numbers (proportions)
for categorical variables. The candidate variables associated with the
risk of cesarean delivery were selected a priori based on the clinical
feature, common sense and predictors assessed in the previously
published literature13-15. Univariable logistics
regression analyses were performed to estimate odds ratios (OR) and 95%
confidence intervals
(CIs). We excluded maternal height
and fetal biparietal diameter as the candidate predictors because of the
multicollinearity with other variables. A backward stepwise elimination
approach was applied to select independent variables for the
multivariable logistics regression model with the Akaike Information
Criterion16.
Nomograms were constructed by selected variables to predict the
probability of cesarean section using statistical software (rms in R;
http:// www.r-project.org). For model performance, we assessed the
discrimination
(the ability to differentiate between the prediction and outcome) and
calibration (the discrepancy between predicted and observed outcomes).
To quantify the model discrimination, we calculated the
concordance (C-) index as
the
area under the receiver operating characteristic curve (AUC). The
calibration was evaluated by calibration plots, accompanied by the
Hosmer-Lemeshow goodness-of-fit test. The model internal validation was
accessed by bootstrapped resampling to quantify
overoptimism17. The optimal cutoff value was
calculated by maximizing the Youden index using receiver operating
characteristic curve (ROC) analysis (i.e., sensitivity + specificity −
1), and then the sensitivity and specificity were estimated. Finally,
decision curve analyses were also applied to evaluate the net benefit of
the prediction model (rmda in R; http:// www.r-project.org)18. All tests were 2-sided, P value less than
0.05 was considered statistically significant. The analyses were
performed using R statistical software, version 4.0.3.