Timing of blastocyst transfer relative to the thaw: a critical factor in outcome following FET

Abstract

Objective: To evaluate effects of different blastocyst freezing and thawing protocols on pregnancy and implantation rate. Design: Retrospective cohort study. Setting: Multi-site, private practice. Patients: Infertile women ages between 27-51 years (n 95) undergoing IVF. Interventions: Women undergoing in vitro fertilization (IVF) between January and December 2002 had surviving expanded supernumerary blastocysts cryopreserved 5 and 6 days following fertilization. All embryos were cultured in P1 (Irvine Scientific) medium until day 3 and thereupon in Blastocyst medium (Irvine Scientific) until day 5 or 6. Patients were allocated to Group 1 or Group 2 based on the method of cryopreservation and the time interval between the thawing process and the frozen embryo transfer (FET). In Group 1, the cryopreservation process involved treatment with 5% and 9% Glycerol 0.2 M sucrose for 10 minutes followed by freezing through exposure of the blastocysts to 7°C for 17 min, and thereupon lowering the temperature by 0.3°C/min to 38°C. Prior to the FET the embryos were thawed in a water bath at 33°C, treated with 0.5 M and 0.2 M sucrose for 10 min and then immediately transferred to the uterine cavity. In Group 2, the same method of exposure of the blastocysts to glycerol and sucrose as described above was employed at 20°C. Thereupon the temperature was progressively reduced by 2°C/min to 6.5°C and maintained at this point for 17 min. Thereafter the temperature was lowered at a rate of 0.3°C/min to 38°C. However, in contrast with Group 1 patients whose blastocysts were transferred immediately upon thawing, the timing of FET’s in Group 2 was predicated upon whether day 5 or day 6 blastocysts were being thawed; whereas day 5 blastocysts were thawed, then treated with 0.5 and 0.2 M sucrose for 10 min, cultured overnight and transferred the following afternoon. Blastocysts frozen on day 6 were thawed similarly on the morning of the scheduled FET, cultured for at least 4 hours and then transferred. Primary measures of outcome: Ongoing pregnancy rate (beyond 12 weeks) and implantation rate (number of viable gestations per transferred blastocyst). Results: The ongoing pregnancy and implantation rates were 16% and 11% for Group 1 as compared to 32% and 20% for Group 2 (P 0.05). Conclusions: Blastocyst transfer has been shown to be associated with improved pregnancy and implantation rates. Recent advances in embryo culture media and techniques have brought about improved embryo quality and survival to the blastocyst stage in the IVF setting. The resulting increase in availability of blastocysts has allowed for an ever increasing degree of discretion in the selection of only the best quality supernumerary blastocysts for cryopreservation. Our data strongly suggests that while the protocol of blastocyst cryopreservation does not affect FET outcome, the timing of the thaw, relative to the transfer could be a critical consideration.