Clinical Characteristics of Unilateral versus Bilateral Ovarian Endometriomas and Risk Stratification in Stage IV Disease Using the rASRM Score: A Retrospective Cohort Study from a Tertiary Medical Center.

Background: Endometriomas are a common manifestation of endometriosis and can present as unilateral or bilateral disease. While bilateral involvement is clinically suspected to reflect more severe disease and greater impairment of ovarian reserve, the specific phenotypic differences and their implications for surgical complexity and fertility preservation remain insufficiently characterized. This study aimed to compare the clinical characteristics of unilateral vs bilateral endometriomas and to explore risk stratification within Stage IV disease using the revised American Society for Reproductive Medicine (rASRM) score. Methods: A retrospective cohort analysis was conducted on 496 patients with histopathologically confirmed endometriomas who underwent laparoscopic surgery at a tertiary medical center between March 2022 and December 2025. A modified intraoperative Endometriosis Fertility Index (EFI), excluding postoperative pregnancy data, was calculated for all patients based on rASRM scores, adnexal functional assessment, and age-related deductions. Clinical, surgical, and pathological data were compared between unilateral (n=372) and bilateral (n=124) groups. A further subgroup analysis was performed on 227 patients with rASRM Stage IV disease identified from the 496-patient cohort, using the median total rASRM score (74 points) as a cutoff to identify a high-risk subgroup. Results: Bilateral cysts accounted for 25.0% of cases. Compared with the unilateral group, patients with bilateral cysts were significantly older (32.31 vs 29.67 years, P<0.001) and presented with more severe disease phenotypes: higher rASRM scores (84.81 vs 51.32, P<0.001), greater incidence of deep infiltrating nodules (83.6% vs 65.8%, P<0.001), larger maximum nodule diameter (1.39 vs 0.99 cm, P<0.001), higher rate of complete cul-de-sac obliteration (61.3% vs 41.4%, P<0.001), lower total antral follicle count (10.16 vs 14.90, P<0.001 after age adjustment), lower modified EFI scores (5.84 vs 7.38, P<0.001 after age adjustment), and a higher proportion of Stage IV disease (88.6% vs 47.8%, P<0.001). In the Stage IV subgroup, patients with rASRM scores ≥74 had longer operative times, lower EFI scores, a higher proportion of bilateral cysts, and more severe Douglas' pouch involvement. Binary logistic regression confirmed that bilateral ovarian cysts and severe Douglas' pouch involvement were key independent predictors for this stratification (P<0.05). Conclusion: Bilateral endometriomas are strongly associated with a more advanced disease phenotype characterized by greater anatomical distortion and lower antral follicle count (AFC), although preoperative AMH levels did not differ significantly between groups. Within Stage IV disease, an rASRM score ≥74-derived as the median of this cohort-identifies a subgroup with higher surgical complexity and reduced fertility potential, providing exploratory, hypothesis-generating stratification insight rather than a validated staging refinement. These findings underscore the need for early fertility counseling in patients with bilateral disease, although external validation of the proposed cutoff is required.