Participants
This retrospective study was conducted in the Department of Obstetrics and Gynecology of The First Affiliated Hospital of Soochow University (tertiary referral center, hospital 1 ) and (secondary referral center, hospital 2 ). For the development cohort, we used data from hospital 1 between January 1, 2011 and August 31, 2017. External validation sampled from hospital 2 between January 1, 2013 to December 31, 2019. Institutional Review Board approval by these two hospitals was obtained for the study waiving informed consent for this retrospective study. Methods and reporting guidelines were followed by the TRIPOD (Transparent Reporting of a multivariable prediction model for Individual Prognosis Or Diagnosis) statement[13] (Additional files 1).
Inclusion and exclusion criteria
Low-risk pregnancy nulliparous women undergoing the labor with singleton, term(37 0/7 weeks of gestation or greater) and cephalic pregnancies were recruited. Exclusion criteria were as follows:
(1) Women had complications during pregnancy(e.g., cardiac failure, severe liver and kidney diseases, hypertensive disorders of pregnancy, diabetes, oligohydramnios, placenta previa, vasa previa and fetal growth restriction);
(2) Women had a scarred uterus(e.g., myomectomy);
(3) Women had contraindications to vaginal delivery;
(4) Women with the cesarean delivery on maternal request.
The pregnant women who meet the criteria were divided into the emergency cesarean delivery group and the vaginal delivery group according to their delivery modes.
Data collection
Data on maternal characteristics and perinatal parameters were collected from the institution’s obstetrics database, which was obtained by the patient’s medical record review.
Characteristics
The outcome of interest was defined as cesarean delivery. A cesarean delivery was performed if there was fetal distress, arrested active phase, prolonged latent phase, prolonged second stage, arrested descent, fever indicating chorioamnionitis, and other medical indications, such as threatened uterine rupture. It is worth noting that we only collected the major indications. The candidate predictor variables had to be easily accessible through characteristics’ data. To identify predictor variables, a systematic review of the literature was conducted[8-12, 14-16]. The following variables were recorded: maternal age, height, weight, baseline body mass index (BMI), weight change during pregnancy, gestational age at delivery, premature rupture of membranes (PROM), epidural analgesia, meconium-stained amniotic fluid, induction methods(oxytocin, amniotomy, disposable cervical dilator balloon, prostaglandin(Propess or Misoprostol)), neonatal sex, and neonatal birth weight.
Operational definitions
The relevant guidelines[17, 18] were used to determine cesarean delivery indications such as arrest of descent and a prolonged second stage of labor. BMI was calculated as weight (kg)/[height (m)]2. Baseline BMI was defined as pre-pregnancy BMI. Gestational age was calculated by the date of the last menstrual period and confirmed by ultrasound examination during first-trimester (by measuring the crown-rump length) or second-trimester(by measuring biparietal diameter, abdominal circumference and femur length). We used the following modes for grouping for induction methods.
(1) Oxytocin Induction group: initial cervical dilation < 6 cm with only oxytocin induction. This was defined as Induction method 1;
(2) Amniotomy group: initial cervical dilation < 6 cm with artificial rupture of membranes with or without oxytocin induction(amniotomy after Prostaglandin E2 and Disposable cervical dilator balloon were not included). This was defined as Induction method 2;
(3) Disposable Cervical Dilator Balloon: with Disposable cervical dilator balloon induction. This was defined as Induction method 3;
(4) Prostaglandin E2 group: with Prostaglandin E2(Propess) induction. This was defined as Induction method 4;
(5) Cervical Dilation≥6cm group: initial cervical dilation≥6cm experience augmentation of labor(experience amniotomy and/or oxytocin);
(6) Unreceived Labor Intervention group: women entered labor naturally and the labor did not be intervened.
Induction group was defined as group (1) plus (2) plus (3) plus (4), augmentation group was defined as group (5). Considering that the women in the augmentation group naturally entered the active stage of labor, the augmentation group and the unreceived labor intervention group were regarded as a group, which serves as a reference group for the induction method (1)-(4). For convenience, spontaneous labor group was defined as group (5) plus group (6).
Statistical analysis
Data analysis was conducted by using the statistical software package SPSS (24.0) and R (3.6.2).
Univariate analysis was performed for all clinical data. For continuous variables, the tests of normality were performed first. The Student’s t-test was used to compare the continuous variables with a normal distribution. The Mann-Whitney test was used to compare discrete or continuous variables without a normal distribution. The chi-square test and Fisher exact test were used, as appropriate, for the categorical variables. Standard descriptive statistics (mean ± standard deviations or median and interquartile range) were used to summarize continuous variables. Percentage and frequency were used for categorical variables.
Baseline variables that were considered clinically relevant or that showed a univariate relationship with outcome(candidate variables with a p value <0.05 on univariate analysis) were entered into the multivariable logistic regression model. Variables for inclusion were carefully chosen, given the number of events available, to ensure parsimony of the final model. The results of logistic regression models were presented as odds ratio (OR) with their 95% confidence intervals (CIs).
The discrimination and calibration of the prediction model were evaluated. Discrimination is the extent to which patients with cesarean delivery is identified likely to have this positive outcome. Calibration refers to the extent to which the calculated risks reflect the actual percentage of women with the outcome in each group. It is the agreement between observed outcomes and predictions. The area under the receiver operating characteristic curve(AUC ROC) was calculated to assess the discrimination ability. AUC ROC was interpreted using following categories: non-informative (AUC ROC = 0.5), poor accuracy (0.5 < AUC ROC <0.7), moderate accuracy (0.7 < AUC ROC <0.9), high accuracy (0.9 < AUC ROC < 1); and perfect accuracy (AUC ROC = 1)[19]. The calibration of the prediction model was assessed using the Hosmer-Lemeshow goodness-of-fit test(P>0.05 was taken to indicate good fitting) and/or calibration plot. Decision curve analysis (DCA) was used to assess the clinical value of the model, which was a method for evaluating the net benefit. We assessed internal validity with a bootstrapping technique (resample 1000 times) to show the performance of model. We conducted an external verification of the final model in another hospital. We reported the predictive performance in the validation cohort also using the measures of discrimination and calibration. A nomogram was established using the predictive model by R. Established a dynamic nomogram by using the rms DynNom, and built a web online applications through shinyapps. All P values were two-tailed, and a significance level of 5% was used.