Vitrification preservation of good-quality blastocysts for more than 5 years reduces implantation and live birth rates

Abstract STUDY QUESTION Does vitrification cryopreservation of embryos for more than 5 years affect the pregnancy outcomes after frozen embryo transfer (FET)? SUMMARY ANSWER Vitrification cryopreservation of good-quality blastocysts for more than 5 years is associated with a decrease in the implantation rate (IR) and live birth rate (LBR). WHAT IS KNOWN ALREADY Previous studies have predominantly focused on embryos cryopreserved for relatively short durations (less than 5 years), yet the impact of extended cryopreservation duration on pregnancy outcomes remains a controversial issue. There is a relative scarcity of data regarding the efficacy and safety of storing embryos for 5 years or longer. STUDY DESIGN, SIZE, DURATION This retrospective study involved 36 665 eligible vitrified-thawed embryo transfer cycles from 1 January 2016 to 31 December 2022, at a single fertility center in China. PARTICIPANTS/MATERIALS, SETTING, METHODS Patients were divided into three groups according to embryo storage time: Group 1 consisted of 31 565 cycles, with storage time of 0–2 years; Group 2 consisted of 4458 cycles, with a storage time of 2–5 years; and Group 3 included 642 cycles, with storage time exceeding 5 years. The main outcome measures were IR and LBR. Secondary outcome variables included rates of biochemical pregnancy, multiple pregnancy, ectopic pregnancy, and miscarriage, as well as neonatal outcomes. Reproductive outcomes were analyzed as binary variables. Multivariate logistic regression analysis was used to explore the effect of preservation time on pregnancy outcomes after correcting for confounding factors. In addition, we also assessed neonatal outcomes, such as large for gestational age (LGA) and small for gestational age (SGA). MAIN RESULTS AND THE ROLE OF CHANCE IRs in the three groups (0–2, 2–5, and >5 years) were 37.37%, 39.03%, and 35.78%, respectively (P = 0.017), and LBRs in the three groups were 37.29%, 39.09%, and 34.91%, respectively (P = 0.028). After adjustment for potential confounding factors, compared with the 0–2 years storage group, prolonged embryo vitrification preservation time (2–5 years or >5 years) did not affect secondary outcomes such as rates of biochemical pregnancy, multiple pregnancy, ectopic pregnancy, and miscarriage (P > 0.05). But cryopreservation of embryos for more than 5 years reduced the IR (adjusted odds ratio (aOR) 0.82, 95% CI 0.69–0.97, P = 0.020) and LBR (aOR 0.76, 95% CI 0.64–0.91, P = 0.002). Multivariate str