Effects of prolonged embryo cryopreservation after fertility treatment in Denmark.

Abstract: This retrospective cohort study examined the effects of the Danish legislative amendment extending the permitted duration of embryo cryopreservation from five years until the woman reaches 46 years on frozen embryo transfer (FET) during assisted reproductive technology. The number of embryos discarded across the eight public fertility clinics due to the five-year storage limit was quantified between January 1, 2017, and December 31, 2019, with an average annual destruction of 1,399 embryos. As of March 2024, 29,591 embryos were cryopreserved in the public fertility clinics. FET cycles from the public clinics between October 2020 and March 2024 were included in the outcome analysis (n = 14,677), of which 1.9% (n = 284) involved embryos cryopreserved >5 years. Ongoing pregnancy rate (OPR) and live birth rate (LBR) were assessed by parity and cryopreservation duration (≤5 years vs >5 years). OPR and LBR increased significantly with parity (36.7 and 31.3% for first, 42.6 and 35.6% for second, and 48.8 and 39.0% for third/fourth child), although the increase in LBR for first vs third/fourth child was not statistically significant. OPR and LBR in FET cycles with embryos cryopreserved >5 years were not significantly different from cycles with embryos cryopreserved ≤5 years (OPR: 36.4 vs 38.0%; LBR: 30.9 vs 32.2%). During the study period, 84 children were born from embryos cryopreserved >5 years that previously would have been destroyed. The findings suggest that the amendment has enabled patients to use surplus embryos for family expansion and prevented the disposal of many good embryos. Lay summary: In fertility treatment, cryopreservation is the process of freezing fertilized eggs called embryos. Embryos can be warmed and transferred to the uterus in a future treatment cycle, giving patients another chance at pregnancy without repeating egg retrieval and fertilization. In Denmark, a recent legislative amendment extended the storage limit for embryos from five years to the woman's 46th birthday. This study investigated how this amendment affected the use of cryopreserved embryos as well as pregnancy and live birth outcomes after FET. Few transfers involved embryos cryopreserved over five years. Pregnancy and live birth rates were similar whether embryos had been frozen under or over five years, and 84 children were born from embryos stored over five years that previously would have been destroyed. The findings suggest that the amendment has enabled patients to use embryos from previous treatments for family expansion and reduced unnecessary destruction of good embryos.