Storage time does not influence pregnancy and neonatal outcomes for first single vitrified high-quality blastocyst transfer cycle

Research question Does blastocyst storage time have an impact on pregnancy and neonatal outcomes following the first single vitrified/warmed high-quality blastocyst transfer cycle for young women? Design Retrospective cohort study in a university-affiliated reproductive medical centre. Results A total of 2938 patients undergoing their first frozen embryo transfer (FET) cycle with a single high-quality blastocyst (Day 5: 3BB and above; Day 6: 4BB and above) transferred were divided into five groups: Group A with storage time ≤3 months (n = 1621), Group B with storage time of 4–6 months (n = 657), Group C with storage time of 7–12 months (n = 225), Group D with storage time of 13–24 months (n = 104), and Group E with storage time of 25–98 months (n = 331). After adjusting for confounding factors by multivariate logistic regression, there were no significant differences in live birth rate [Group A as reference; Group B: adjusted odds ratio (aOR) 0.954 (95% CI 0.791– 1.151); Group C: aOR 0.905 (95% CI 0.674–1.214); Group D: aOR 0.727 (95% CI 0.474–1.114); Group E: aOR 1.185 (955 CI 0.873–1.608)], β-human-chorionic-gonadotropin-positive rate, clinical pregnancy rate and miscarriage rate between Group A and the other groups. Among all singletons born after FET, there were no significant differences with regards to gestational age, preterm birth, birthweight, low birthweight, high birthweight and macrosomia. Conclusion Long-term cryostorage of human vitrified high-quality blastocysts does not affect pregnancy or neonatal outcomes.