Vitrification and Slow Freezing for Cryopreservation of Blastocyst

Improvement in the treatment of infertility by Assisted Reproductive Techniques (ART) allows single embryo transfer to be applied without compromising pregnancy rates after the first in vitro fertilization (IVF) or Intra Cytoplasmic Sperm Injection (ICSI) attempt in women less than 36 years old with good embryo quality. The policy of transferring more than one embryo after IVF or ICSI has been the main reason for the numerous twin or triple pregnancies reported in Europe and United States over the past 15 years. These multiple pregnancies are the main disadvantage of ART because of their negative impact on obstetrical, neonatal and economic outcome. In the past, embryos were replaced in the uterus on either Day-2 or 3 of development at the cleavage stage. With the development of physiologically-based sequential culture media, it has also been suggested that extending embryo culture to Day-5 in order to transfer the embryo at the blastocyst stage would enhance the likelihood of pregnancy. Nevertheless, it has been observed higher pregnancy rate after the transfer of fresh blastocyst but not after the transfer of thawed blastocyst frozen by slow freezing procedure. However a recent embryo freezing technique (vitrification) seems to show significant higher pregnancy rates when blastocyst are frozen by this method. To our knowledge, no publications have reported the outcome of single embryo transfer at blastocyst stage by a prospective randomized and comparative study including the results of fresh and frozen/thawed blastocyst by these two methods (slow freezing vs. vitrification) in case of single embryo transfer . Therefore, the aim of our study is to analyze whether extended culture of Day-2 top embryos to blastocyst-stage may improve the cumulative delivery rate in an in vitro fertilization program with Single Embryo Transfer policy in a prospective and randomized study integrating the transfer of fresh and frozen/thawed embryos using a slow freezing versus vitrif