Obesity and morbid obesity are associated with reductions in live birth rate and increased miscarriage rate after frozen embryo transfer

PURPOSE: . This study assessed BMI's impact on frozen embryo transfer (FET) outcomes. METHODS: A retrospective cohort study (2017-2024) of 1,424 FETs from 889 patients, stratified by BMI: normal (18.5-24.9), overweight (25.0-29.9), obese (30.0-39.9), morbidly obese (≥ 40). Primary outcome was live birth rate per transfer (LBR). Secondary outcomes include clinical intrauterine pregnancy (CIUPR), miscarriage rates (MR), and proportion achieving ≥ 1 live birth. Outcomes were adjusted for age, AMH, FSH, PCOS, tubal infertility, and race. RESULTS: LBR decreased with increasing BMI for all transfers (53.5%, 52.8%, 42.5%, 30.8%, p < 0.001) and single elective euploid transfers (SEETs) (54.2%, 57.8%, 44.5%, 32.3%, p < 0.001). CIUPR showed the same pattern (58.0%, 59.5%, 50.4%, 39.7%; p < 0.001 all transfers; 58.6%, 63.1%, 51.0%, 40.3%; p < 0.001 SEETs), as did the proportion of patients achieving at least one birth (72.9%, 68.3%, 65.8%, 51.1%; p = 0.020). MR significantly increased with BMI (7.4%, 11.2%, 15.2%, 22.6%, p<0.001) for all transfers but not for SEETs (7.5%, 8.4%, 12.8%, 20.0%; p = 0.169). Each one-unit increase in BMI was associated with a 3% reduction in odds of live birth (aOR 0.969, 95% CI 0.951-0.987, p<0.001). CONCLUSION: Increasing BMI is associated with lower LBR and CIUPR, following FET, including SEETs. MR increased with BMI for all FETs, with a similar but non-significant gradient among SEETs. This highlights the importance of considering BMI when counseling patients, particularly those with morbid obesity.