Does embryo cryopreservation duration impact embryo thaw survival and live birth outcomes for euploid embryos?

To evaluate whether the duration of embryo cryopreservation is associated with survival after warming and live birth rates (LBR) following single euploid blastocyst transfer. We conducted a retrospective cohort analysis of 3,895 embryo warming cycles between 2017-2021 within four member clinics part of a large fertility network. All first, single embryo transfers of euploid blastocysts were included. Endometrial preparation was performed in natural or medicated cycles per physician discretion. Duration of cryopreservation for each warming cycle was calculated by date of embryo warming minus date of embryo freezing. Primary outcomes were embryo survival rate per warming cycle and live birth rate per transfer. Modified Poisson regression modeling was used to evaluate the association between duration of cryopreservation (<1 year, 1-1.9 years, 2-2.9 years, and 3-3.9 years) and pregnancy outcomes while adjusting for age at cryopreservation and body mass index. Relative risks (RR) and 95% Confidence Intervals (CI) were calculated. In total, 3,895 embryo warming cycles were included with 3,667 frozen embryo transfers. Overall, there was no significant association between duration of cryopreservation and embryo survival rate (<1 year: 97.8%, 1-1.9 years: 95.5%, 2-2.9 years: 97.4%, 3-3.9 years: 97.5%; Table 1). Additionally, there was no significant association between duration of embryo cryopreservation and live birth (< 1 year: 51.1%, 1-1.9 years: 53.7%, 2-2.9 years: 62.1%, 3-3.9 years: 55%) following single euploid embryo transfer. In this cohort, duration of embryo cryopreservation was not associated with rates of embryo survival at the time of warming or live birth after single euploid embryo transfer.