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

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.

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.

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.

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. Similar content being viewed by others Abbreviations - UFA: - Uterine flexion angle - UCFA: - Uterine corpus flexion angle - DIE: - Deep infiltrating endometriosis - BMI: - Body mass index - TVUS: - Transvaginal ultrasonography - rASRM: - Revised American Society for Reproductive Medicine - ICC: - Intraclass correlation coefficient - ROC: - Receiver operating characteristic - AUC: - Area under the curve - CI: - Confidence interval - IQR: - Interquartile range - POD: - Pouch of Douglas

Not applicable. Funding Dongguan Science and Technology of Social Development Program; No.: 20231800904322. Author information Authors and Affiliations Corresponding author Ethics declarations Ethics approval and consent to participate The study was performed in line with the principles of the Declaration of Helsinki. Approval was granted by the Medical Ethics Committee of Dongguan People's Hospital, which is affiliated to The Tenth Affiliated Hospital, Southern Medical University (Dongguan People's Hospital), before the study began. The endometriosis patients’ right to be informed about the study was ensured, and all patients provided written consent to participate. For the two control groups, consent to participate was waived by this same ethics committee due to retrospective data collection and anonymized analysis. Consent for publication Not applicable. Competing interests The authors declare no competing interests. Additional information Publisher’s Note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Supplementary Information Rights and permissions Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/. About this article Cite this article Chen, J., Zhong, S., Liang, J. et al. The uterine corpus flexion angle: a novel and objective sonographic marker for endometriosis severity and the quantification of the question mark sign. BMC Women's Health (2026). https://doi.org/10.1186/s12905-026-04695-6 Received: Accepted: Published: DOI: https://doi.org/10.1186/s12905-026-04695-6