Triple product of diameters predicts early medical abortion failure better than other gestational sac metrics in outpatients.

Early medical abortion (EMA) with mifepristone-misoprostol commonly relies on subjective metrics (e.g., last menstrual period-derived gestational age) for failure risk assessment, which may introduce uncertainty. This retrospective study aimed to identify objective predictors of EMA failure and explore the utility of different gestational sac size metrics for clinical risk stratification. Early pregnancy outpatients who underwent medical abortion with the mifepristone-misoprostol regimen at Karamay Central Hospital (Xinjiang, China) from December 2024 to July 2025 were included in our study. EMA failure was defined as the need for surgical evacuation. Analyses included univariate assessment, logistic regression, and performance evaluation of gestational sac size metrics. After exclusion, 159 women were included in the final analyses. The overall EMA success rate was 89.31% (142/159). Maternal age ≥35 years was associated with higher failure risk (OR = 2.82, 95% CI, 1.01–7.89, p = 0.048). Gestational sac size emerged as an important objective correlate of EMA failure; among five assessed metrics (ellipsoid volume, maximum diameter, mean diameter, sum of three diameters, triple product of diameters), the triple product of diameters showed the highest correlation with failure (r = 0.316) and a significant association with failure risk (OR per 1,000 mm3 increase = 1.08, 95% CI, 1.03–1.14, p = 0.003). The triple product model demonstrated good discrimination, with an area under the curve (AUC) of 0.78 (95% CI, 0.68–0.88). Calibration was adequate (Hosmer-Lemeshow p = 0.62), and internal validation using bootstrap resampling confirmed stable performance (optimism-corrected AUC 0.76, 95% CI, 0.65–0.86). Its optimal cutoff (3648) yielded sensitivity = 0.765, specificity = 0.688, and Youden Index = 0.453. Fetal heart activity, embryonic bud presence, and parity were not significantly linked to failure (all p > 0.05). Gestational sac size is an important objective indicator for predicting mifepristone-misoprostol EMA failure, and among various gestational sac size metrics, the triple product of diameters demonstrates the highest predictive value.