Blastocyst transfer after aseptic vitrification of zygotes: an approach to overcome an impaired uterine environment

In some IVF cycles, no fresh embryo transfer in the stimulated cycle is advisable. The cryopreservation of zygotes and the
transfer of blastocysts in a cryo-embryo transfer is an option to circumvent an inadequate uterine environment due to risk of ovarian
hyperstimulation syndrome, inappropriate endometrium build up, endometrial polyps or uterine myomas. For this strategy, highly
secure and safe cryopreservation protocols are advisable. This study describes a protocol for aseptic vitrification of zygotes that
results in high survival rates and minimizes the potential risk of contamination in liquid nitrogen during cooling and long-term storage.
In mouse zygotes, there was no difference in efficiency as compared with a conventional open vitrification system. In IVF
patients, aseptically vitrified zygotes showed no difference in blastocyst formation rate as compared with sibling zygotes kept in
fresh culture. A clinical study comprising 173 cryo-cycles with a transfer of blastocysts originating from vitrified zygotes showed
an ongoing pregnancy rate of 40.9%. The live birth rate per patient was 36.8%. A combination of good clinical results and increased
safety conditions due to aseptic vitrification encourages the use of cryo-embryo transfer for patients with a suboptimal uterine environment
in a fresh cycle.