Etiology of thin endometrial lining and euploid single embryo transfer outcomes

Research question: Do frozen euploid single embryo transfer (SET) outcomes differ in patients with thin endometrial lining based on the cause of thin endometrium? Design: Single-centre retrospective cohort study including patients with thin endometrial lining (<7 mm) undergoing euploid SET between 2016 and 2024. Patients were grouped by cause of thin lining: ‘thin-surgery’ (history of uterine surgery) and ‘thin-constitutional’ (no prior uterine surgery). Reference groups were patients with normal endometrial thickness (≥7 mm) who underwent previous uterine surgery (normal-surgery) and those who did not (normal-constitutional), matched (3:1) by oocyte age, body mass index and treatment year. Primary outcome was live birth. Secondary outcomes included pregnancy, biochemical and clinical pregnancy loss rates. A subgroup analysis evaluated surgical patients with Asherman’s syndrome. Results: A total of 272 cycles from 218 patients were included: 120 cycles (n = 89 patients) in the thin-surgery cohort and 152 cycles (n = 129 patients) in the thin-constitutional cohort. Live birth was lower in the thin-surgery compared with the thin-constitutional cohort (25.8% [31/120] versus 44.7% [68/152], P = 0.001, adjusted OR 0.51, 95% CI 0.29 to 0.91, P = 0.02). Both thin lining cohorts had lower live birth rates compared with reference groups: thin-surgery (25.8% [31/120]) versus normal-surgery 49.2% [177/360], P < 0.001) (OR 0.32, 95% CI 0.20 to 0.54, P < 0.001); thin-constitutional (44.7% [68/152]) versus normal-constitutional 55.3% [252/456], P = 0.04), aOR 0.66, 95% CI 0.44 to 0.97, P = 0.01). Although within the thin-surgery cohort, a lower live birth rate was observed in patients with Asherman’s syndrome than those without (19.2% [9/47] versus 30.1% [22/73], aOR 0.51 95% CI 0.23 to 1.13, P = 0.09); this failed to reach statistical significance. Conclusion: Patients with thin endometrium who underwent previous uterine surgery had lower live birth rates after euploid SET compared with those with constitutionally thin lining. The cause of thin endometrial lining may serve as a valuable prognostic factor when counselling and managing patients with persistently thin lining.