Freeze-all versus fresh embryo transfer strategies in low-prognosis women: an emulated target trial study.

RESEARCH QUESTION: How effective is a freeze-all strategy in low-prognosis women, as defined by the Patient-oriented Strategies Encompassing Individualized Oocyte Number criteria, in terms of the cumulative live birth rate in comparison with a fresh embryo transfer strategy? DESIGN: A target trial was emulated using a retrospective cohort of 2102 women undergoing their first or second cycle of IVF/intracytoplasmic sperm injection (ICSI) at a tertiary reproductive centre. Participants were divided into a freeze-all group (n = 912) and a fresh embryo transfer group (n = 1190). To emulate randomization and control for confounding, propensity score overlap weighting was applied to balance 20 baseline covariates. The primary outcome was cumulative live birth per initiated cycle. RESULTS: After propensity score overlap weighting, the cumulative live birth rate was lower in the freeze-all group than in the fresh embryo transfer group (40.0% versus 46.7%), with a pooled risk difference of -6.69% (95% CI -7.12% to -6.25%). Live birth (risk difference = -3.94%) and clinical pregnancy (risk difference = -4.05%) rates after the first transfer were also lower in the freeze-all group. Results were consistent across sensitivity analyses. Subgroup analyses indicated that the benefit of fresh transfer was more pronounced in women aged ≥35 years and women with four or more oocytes retrieved. CONCLUSIONS: In low-prognosis women, a fresh embryo transfer strategy was associated with a higher cumulative live birth rate than a freeze-all strategy. These findings support a prognosis-tailored, individualized approach to embryo transfer rather than a uniform policy.