Randomized Comparison to Freeze Human Embryos by Either Vitrification or Slow Freezing Protocols

Despite all the advances achieved in vitro fertilized treatments, it is known that the chances of implantation for fertilised in vitro embryo remain limited, at around 20%. Cryopreservation of supernumerary embryos produced during IVF cycles provides an opportunity for patients to increase the number of transfers per oocyte harvest cycle, increasing then their chances of conception.

The freezing technique may involve different media and methods, which lead to different survival and developmental rates after thawing.

Vitrification is a new freezing method, which consists in exposing cells to high concentrations of cryoprotectants and then cooling them ultra-rapidly; this induces an increase in viscosity which favours the formation of a vitreous state without any ice crystals formation. This method contrasts with the slow freezing method (currently employed), which is based on exposure to very low concentrations of cryoprotectants combined with long cooling times and associated with a higher risk of ice crystal formation.

Both methods can reduce cell survival and embryo ability to develop, either by the high concentrations of cryoprotectants in case of vitrification, or by ice crystals formation in case of slow freezing. The advantages of vitrification in embryology may be considerable because embryos seem more sensitive to ice crystal formation than to cyroportectant concentration; consequently, the elimination of office crystal injury may increase their survival chances. Additionally in the case of vitrification, the time required for equilibration and cooling is considerably reduced as well as the need for expensive equipment such as programmable machine is eliminated.

At present, vitrification and slow freezing are used for the cryopreservation of oocytes and embryos at all stages of development. Encouraging results have been obtained with vitrification, but no study has randomly compared in one study, the two protocols and cryoprotectors. The purpose of this clinic randomised study is to compare three treatments: traditional slow freezing method currently employed (control group) and two different commercial vitrification methods (experimental groups) to assess the efficacy that this technique may involve.