Clinical data were collected from 96 pregnant women diagnosed with anterior asynclitism from January 2017 to January 2023 (observation group), and 96 pregnant women diagnosed with persistent occipitotransverse position during the same period, selected in chronological order (control group). As the capital of Sichuan Province, Chengdu holds a leading position in regional economic development and possesses relatively abundant medical and healthcare resources. However, there are still differences in resource allocation and technical levels among hospitals at different levels within the region. The hospitals involved in this study are district-level general hospitals, serving the population in the surrounding urban areas and in some rural areas.

Inclusion criteria: full-term singleton pregnancy with indications for trial of vaginal delivery. Exclusion criteria: contraindications to a trial of vaginal delivery. The inclusion and exclusion criteria are applicable to both the observation group (pregnant women diagnosed with anterior asynclitism) and the control group (pregnant women diagnosed with persistent occipitotransverse position).

When screening the pregnant women in the observation group, 96 pregnant women diagnosed with anterior asynclitism were selected through the retrieval of the electronic medical record system from those who met these criteria. When selecting the pregnant women in the control group, also based on these criteria, 96 pregnant women diagnosed with persistent occipitotransverse position were selected from the pregnant women who visited these two hospitals during the same period in chronological order. During the screening process, the operations were carried out strictly in accordance with the established inclusion and exclusion criteria. This study is a case-control study with 1:1 sampling. The control group (persistent occipitotransverse position) was matched to the observation group (anterior asynclitism) for full-term singleton pregnancy, eligibility for trial of vaginal delivery, and exclusion of contraindications to vaginal delivery (same inclusion/exclusion criteria applied to both groups, Table 1).

Since this study involves a comparison of the rates between two groups, the corresponding sample size calculation formula was used. Referring to the estimation method for the rates of two samples, and combining with previous research and clinical experience, the incidences of neonatal asphyxia in cases of anterior asynclitism and persistent occipitotransverse position were predicted to be 15% and 5% respectively [9, 10, 11]. With the two-sided test level set at $\alpha$ = 0.05 and the test power at 0.80, sample size was calculated using the formula “n = [(Z$\alpha$/2 + Z$\beta$)² $\times$ (p1 (1 – p1) + p2 (1 – p2))] / (p1 – p2)²”, where Z$\alpha$/2 = 1.96, Z$\beta$ = 0.84, p1 = 0.15 (predicted neonatal asphyxia rate in the anterior asynclitism group), and p2 = 0.05 (rate in the control group), yielding approximately 86 cases required for each group. Considering the possibility of loss to follow-up or missing data, 96 cases were ultimately included in each group [12]. All cases had complete data for analysis, with no exclusions due to missing information.

Anterior asynclitism was diagnosed through vaginal examination. The sagittal suture was located on the transverse diameter of the pelvic inlet and close to the sacral promontory, posteriorly [14]. Currently, there is no clear quantitative standard for diagnosing anterior asynclitism based on the displacement of the sagittal suture in clinical practice. In this study, the diagnosis of anterior asynclitism was mainly determined by combining the position of the sagittal suture, the condition of the pelvis, the manifestations of the cervix and the fetal head. The anterior parietal bone first entered the pelvis and was impacted behind the pubic symphysis. The posterior part of the pelvis was empty. The anterior lip of the cervix was edematous due to compression. A cephalhematoma might occur on the compressed side of the fetal head (anterior parietal bone). After the surgery, the diagnosis could be further confirmed according to the examination of the location of the cephalhematoma of the newborn. For example, when anterior asynclitism occurred in the left occipitotransverse position, the cephalhematoma was located on the right parietal bone, and when it occurred in the right occipitotransverse position, the cephalhematoma was located on the left parietal bone. In developed regions or hospitals, ultrasound can be utilized during labor to dynamically monitor the inclination angle of the fetal head and its engagement within the pelvis [15]. The angle between the sagittal suture and the midline of the pelvis can be measured to evaluate the degree of inclination. However, most hospitals in China, including our own, lack the capability for ultrasound monitoring during the labor process. Therefore, in this study, diagnosis was mainly based on the clinical manifestations of pregnant women and confirmed through vaginal examinations conducted by experienced obstetricians.

Clinical data information (Table 1): Age, gestational weeks, history of vaginal delivery, BMI, and pelvic assessment. High-risk factors (Table 2): Macrosomia, obesity, pendulous abdomen, umbilical cord around neck, amniotic fluid index 8 cm, and PROM. Clinical manifestations (Table 3): Urinary retention, PROM, cervical edema, fetal head edema, skull overlap, and threatened uterine rupture [16].

Statistical analysis was performed using IBM SPSS Statistics 26.0 (IBM Corp., Armonk, NY, USA). Before conducting the statistical analysis, a normality test was first performed on the measurement data. The Shapiro-Wilk test was used to determine whether the data conformed to a normal distribution. If the data passed the normality test (p $>$ 0.05), it was considered that the measurement data conformed to a normal distribution. Such data were expressed as $\overline{X}$ $\pm$ s, and the t-test was used for the comparison between groups. If the data did not pass the normality test (p $\le$ 0.05), it was regarded as a non-normal distribution. In this case, the Mann-Whitney U test (Wilcoxon rank-sum test for two independent samples) was used to compare measurement data between groups. Categorical data were expressed as counts and percentages [n (%)], and the chi-square ($\chi$²) test was used for the comparison between groups. A p-value 0.05 indicated a statistically significant difference. A multiple logistic regression model was used to analyze the independent risk factors for anterior asynclitism. This study was an exploratory analysis, preliminarily demonstrating the trend of association among various factors. The obtained p-values were not corrected for multiple testing. When interpreting the results of this study, the limitation of uncorrected p-values should be fully considered.

There were no statistically significant differences in the gestational weeks, history of vaginal delivery of the pregnant women between the two groups (p $>$ 0.05). Significant differences were observed in age between the two groups (p 0.05) (Table 1).

When comparing the high-risk factors of the two groups of pregnant women, the incidences of macrosomia, obesity, and pendulous abdomen in the observation group are all higher than those in the control group, and the differences are statistically significant (p 0.05). However, there were no statistically significant differences in the amniotic fluid index and umbilical cord around the neck (p $>$ 0.05) (Table 2).

From the perspective of clinical manifestations, the incidences of PROM, cervical edema, fetal head edema, and threatened uterine rupture were all significantly higher in the observation group than in the control group (p 0.05). However, the difference in the incidence of urinary retention was not statistically significant (p $>$ 0.05) (Table 3).

Age, macrosomia, obesity, pendulous abdomen, and PROM are identified as independent risk factors for anterior asynclitism (p 0.05) (Table 4).

Obesity (BMI $\ge$30 kg/m²) was an independent risk factor for anterior asynclitism (OR = 2.86), consistent with Malvasi et al. [8] (OR = 2.5). Hung et al. [17] reported a weaker association (OR = 1.8) in Asian population, likely due to BMI criteria differences (World Health Organization [WHO] $\ge$30 vs. Asian population $\ge$28 kg/m²). Higher obesity prevalence in cases (15.7% vs. 6.3%) highlights the need for enhanced prenatal nutrition interventions in regional hospitals.

Pendulous abdomen demonstrated the strongest association (OR = 6.49), a novel finding rarely emphasized in prior studies. This may stem from altered pelvic inclination forcing asynclitic engagement, suggesting that prenatal postural assessments (e.g., abdominal circumference/uterine height ratio) [18] could enhance screening accuracy.

Maternal age showed a significant association with anterior asynclitism (OR = 1.13, 13.0% risk increase per year). Despite modest effect size, the higher mean age in the case group (29.9 $\pm$ 3.2 years vs. 28.3 $\pm$ 4.1 years) highlights cumulative risk. These findings align with Peled et al. [2], linking age-related pelvic ligament laxity to altered fetal head rotation.

PROM significantly increased risk (OR = 2.51), potentially due to reduced amniotic fluid volume promoting mechanical obstruction [19]. However, Nahaee et al. [3] found no association with cephalic dystocia, possibly due to our inclusion of earlier PROM cases (6 hours pre-labor), highlighting the need for time-stratified analyses.

In contrast to studies in developed countries, all cases in this study were diagnosed based on vaginal examination rather than intrapartum ultrasound.

While Gimovsky [20] held that prenatal ultrasound has been shown to be more accurate than vaginal examination in describing fetal positional abnormalities, this study found high consistency between empirical diagnosis (e.g., cervical edema, fetal head hematoma location) and postoperative confirmation. These findings suggest that vaginal examination remains viable in resource-limited settings—although the potential for underdiagnosis of mild cases exists. Malvasi et al. [21] improved early diagnosis by 20% using an artificial intelligence algorithm (AIDA system) with ultrasound data, suggesting technology integration for grassroots practice.

The 100% cesarean rate in this study exceeded tertiary hospital rates [19], possibly reflecting limited capacity for complex vaginal delivery in regional hospitals. This highlights the necessity of establishing a regional referral system to ensure that high-risk cases are handled at centers with the necessary conditions.

Neonatal adverse outcomes associated with anterior asynclitism (e.g., intracranial hemorrhage 10.4%) were correlated with abnormal fetal head compression and delivery difficulty. In this study, 75% of cesarean sections (72 cases) involved difficult fetal head extraction, a rate higher than the 50% reported by Alves et al. [22]. This discrepancy may reflect limited surgical experience with deeply impacted heads in regional hospitals.

To reduce complications, a three-tier prevention approach is proposed:

Primary prevention: Prenatal screening for obesity, pendulous abdomen, and macrosomia; implement weight management via nutritional counseling and exercise.

Secondary prevention: Monitor for cervical edema and arrest of fetal descent during labor; use portable ultrasound (if feasible) as an adjunctive diagnosis tool to support timely referral.

Tertiary prevention: Integrate anesthesia and neonatology teams during cesarean delivery; adopt modified fetal head extraction techniques (e.g., reverse fundal pressure) to shorten operative time to 30 minutes [23].

This study offers valuable insights based on a single-center cohort and comprehensive clinical and postoperative diagnosis, with multivariate analysis to control for confounders to strengthen risk factor reliability. The innovation lies in revealing the unique challenges of the management of anterior asynclitism in district-level hospitals. However, as a retrospective study, several limitations must be acknowledged: single-center design and medical record reliance may introduce selection bias (e.g., misclassification or underdiagnosis); lack of multiple testing correction; and absence of lateral asynclitism analysis [24]. Additionally, the study primarily uses traditional clinical diagnosis, missing intrapartum ultrasound, a more accurate tool for real-time assessment of fetal position. Future research integrating intrapartum ultrasound could enhance early detection and management, addressing a key obstetric practice gap [25].