Purpose: The aim was to investigate if improved survival rates could be achieved using a new formulation of solutions for slow freezing of human cleavage stage embryos.
Methods: The evaluation was divided into two parts. The first part was a retrospective analysis of results obtained after freezing and thawing of day 3 embryos from 400 women using an old formulation of cryopreservation solutions compared to results from 108 women for which cryopreservation had been performed using new compositions of solutions. The second part was prospective, adding cycles until similar numbers of patients had been included in both groups. In total, 2274 embryos from 897 patients were thawed using the old formulation of solutions while 1273 embryos from 542 patients were frozen and thawed using the new solutions. The primary endpoint was survival rate.
Results: With the new solutions, the survival rate increased from 82.1 to 94.4 % and the complete embryo survival rate increased from 54.9 to 81.3 %. The implantation rate, clinical pregnancy rate per embryo transfer, and per cycle were 28.2, 45.2, and 43.7 %, respectively, using the old formulations of cryosolutions. With the new solutions, the results reached 33.7, 54.1, and 54.1 %, respectively. All differences in results were statistically significant. The number of cancelled embryo transfers due to no survived embryos was 18 with the old solutions and 0 using the new solutions.
Conclusion: With the new composition of solutions for slow freezing and thawing of embryos, significantly improved results were obtained. Additionally, the number of cancelled embryo transfers was reduced.
Keywords: Cryopreservation; Human embryo freezing; Slow- freezing; Survival rates.