FROZEN IN TIME: INTERVAL OF CRYOPRESERVATION DOES NOT AFFECT REPRODUCTIVE POTENTIAL OF VITRIFIED BLASTOCYSTS.

Many IVF programs have adopted a “freeze-all” approach to embryo transfers (ET) due to the benefits of being able to perform preimplantation genetic testing (PGT), improve endometrial receptivity, and embryo bank. Once embryos are cryopreserved, they should retain the same reproductive potential that they did on the day they were vitrified. However, no large scale studies have confirmed that blastocysts frozen for long periods of time have equivalent pregnancy outcomes to those that are thawed for transfer in the subsequent cycle. This study aimed to compare pregnancy outcomes from embryos frozen for varying intervals to determine whether they have equivalent reproductive potential. This study was a retrospective analysis of all autologous frozen ET cycles at a single institution between 2016 and 2021 with intention for single euploid ET. Patients with prior fresh ETs, those with endometrial thickness <7mm prior to transfer, and those with more than one failed transfer were excluded. Cycles were categorized by the length of the interval the embryo was frozen prior to transfer: 0-3 months, 3-6 months, 6-12 months, 12-24 months, and >24 months. Embryo survival, implantation, clinical pregnancy, clinical pregnancy loss, and ongoing pregnancy rates were calculated and logistic regression controlling for patient age at retrieval was used to compare groups. 7,655 ET cycles were included, of which 7,542 had complete pregnancy data (Table 1). Overall embryo survival was 99.6% and did not differ between groups. After controlling for age at retrieval, there were no differences in implantation, clinical pregnancy, clinical pregnancy loss, and ongoing pregnancy amongst the five groups. Embryos frozen for longer intervals have equivalent reproductive potential to those that remain frozen for brief intervals. Patients may be reassured that embryo banking is an efficient strategy for building their desired family size.