Optimal day to thaw day 5 vitrified blastocysts for frozen embryo transfers

Abstract

Despite blastocyst cryopreservation having become routine practice in embryology labs, the optimal time to thaw a blastocyst has not been well studied in the literature. Practices differ between institutions as day 5 embryos may be thawed the day prior or the day of transfer. The aim of this study was to evaluate if clinical outcomes differ between day 5 blastocyst stage vitrified embryos thawed on the day prior to versus the day of transfer. Design: Retrospective study. Setting: Academic fertility center. Interventions: Blastocysts vitrified and thawed the day prior to transfer (Group 1) were compared to those thawed the day of transfer (Group 2). All embryos were vitrified on the cryolock device and all patients underwent endometrial priming. Pregnancies were initially detected by serum β-human chorionic gonadotropin (hCG) concentrations. Clinical pregnancy and implantation were confirmed by transvaginal ultrasound (US). Exclusion criteria included: donor oocyte, pre-implantation genetic testing, and gestational carrier cycles, as well as cycles that included embryos imported from different centers and cycles with combined day 5 (D5) and day 6 (D6) frozen embryo transfers. Outcome Measures: Positive pregnancy test (serum β-hCG concentration >6 IU/L) per cryothaw cycle, clinical pregnancy (CPR) and implantation rates (number of sacs observed on US over the number of embryos transferred). Statistics: X2-square and t-test were used as appropriate. P<0.05 was considered statistically significant. A total of 112 thawed blastocysts (88 D5, 24 D6) from 94 patients were included in the analysis. Patient characteristics did not differ between the 2 groups of embryos (Table 1). When all blastocyst stage embryos (D5 and D6) were compared between the Group 1 and Group 2, positive pregnancy test (61.7% vs 70.2, p=0.51, respectively), CPR (61.7% vs 62.2%, p=1.00, respectively) and, implantation (85.0% vs 82.4%, p=0.76, respectively) rates did not differ. When the analysis was restricted to D5 embryos, which were the blastocysts affected by the intervention implemented, positive pregnancy test, CPR and implantation rates were similar amongst the groups (Chart 1). According to our findings, thawing D5 blastocysts the day prior to transfer does not impact CPR or implantation rates, thus allowing embryologists some flexibility to the timing of D5 embryo thaw.