The impact of post thaw embryo culture (extended culture) on frozen embryo transfer outcomes: a systematic review and meta-analysis

To systematically evaluate the impact of post-thaw culture duration on frozen embryo transfer (FET) outcomes across different embryonic developmental stages.

We conducted a systematic review and meta-analysis following PRISMA. A comprehensive search of Medline, Embase, Web of Science, and Cochrane identified 14 eligible studies (N = 17,415) utilizing vitrification. The analysis investigated four post-thaw protocols (extended culture 18–48 h), stratified by embryonic developmental stage: (A) D2ext: day 2 extended vs. day 2 (3 studies, N = 879), (B) D3ext: day 3 extended vs. day 3 (3 studies, N = 11,204), (C) D3-to-D5/6: day 3 extended to day 5/6 vs. day 5/6 (4 studies, N = 1449), and (D) D5/6ext: day 5/6 extended to day 5/6 (4 studies, N = 3883). Pooled relative risks (RRs) with 95% confidence intervals (CIs) and heterogeneity were calculated using random-effects models.

Extended culture significantly improved outcomes for day-3 embryos. D3ext increased live birth rates (RR 1.076, 95% CI 1.034–1.119), while D3-to-D5/6 showed the most pronounced benefit (RR 1.229, 95% CI 1.008–1.499) and reduced biochemical pregnancy loss. Conversely, D2ext showed significantly lower clinical pregnancy rates than immediate transfer (RR 0.689, 95% CI 0.569–0.835). For established blastocysts (D5/6ext), extended culture provided no significant improvement in live birth rates.

Post-thaw culture strategies should be tailored to embryonic developmental stage. Our most notable finding indicates that day-2 embryos achieve significantly better outcomes with immediate transfer (2–6 h) rather than prolonged in vitro culture. Conversely, day-3 embryos benefit significantly from extended culture (18–48 h), particularly when cultured to the blastocyst stage. Extended culture offers no clinical advantage for embryos already at the blastocyst stage. Post-thaw culture strategies should therefore be strictly tailored to the specific embryonic developmental stage at the time of cryopreservation.