The uterine corpus flexion angle: a novel and objective sonographic marker for endometriosis severity and the quantification of the question mark sign

Abstract Background This study aimed to investigate the correlation between the uterine corpus flexion angle (UCFA) and the severity of endometriosis, evaluate its clinical utility as a novel objective sonographic parameter, and provide an angular definition for the question mark sign. Methods This mixed prospective-retrospective case–control study enrolled participants in a 1:1:1 ratio into three groups: the endometriosis group, the benign control group, and the normal control group. All participants underwent transvaginal ultrasonography (TVUS). The UCFA and uterine flexion angle (UFA) were independently measured in the standard midsagittal plane by two blinded sonographers. The subjective question mark sign was assessed by sonographers. In the endometriosis group, pelvic adhesions were graded intraoperatively using the revised American Society for Reproductive Medicine (rASRM) criteria. The correlation of the UCFA with the severity of endometriosis and the question mark sign was evaluated via Spearman’s correlation, multivariate linear regression, and receiver operating characteristic curve analyses. Results The UCFA progressively increased with higher rASRM stage ( P < 0.001) and greater pelvic adhesion severity ( P < 0.001). Multivariate linear regression analysis revealed that UCFA was primarily determined by pelvic adhesion score ( β = 0.441), whereas UFA was predominantly influenced by baseline uterine position ( β = 0.508). Subgroup analysis in 166 adenomyosis-free endometriosis patients confirmed that adhesion score remained the primary predictor of UCFA ( β = 0.457). The optimal cutoff for the sonographic question mark sign was UCFA ≥ 220°, with an area under the curve (AUC) of 0.958. UCFA values were relatively stable in women without endometriosis, with a median of 179.0° in the benign control group and 178.0° in the normal group. Conclusions The UCFA is a reliable objective sonographic marker for assessing the severity of endometriosis-related pelvic adhesions. The proposed criterion of UCFA ≥ 220° provides a validated, quantitative definition for the question mark sign, offering a practical tool for standardized ultrasonography assessment and preoperative surgical risk stratification.