Vitrification vs Slow Embryo Freezing

By Triantafyllos Triantafyllou
Clinical Embryologist, Embryology Lab Director, IVF MITERA

Until recently, embryo cryopreservation was seen as a necessary compromise. Despite the fact that the first child born from a frozen embryo was back in 1983, the success rate of embryo transfers after cryopreservation and thawing were much lower compared to those using “fresh” embryos. It was thus a given that fertilization must be followed by embryo transfer on the same cycle, while frozen embryos were preserved for a second chance, but with a lower success rates.

Intuitively, this seems logical, given that everyday experience has shown that cryopreserved cells usually have a lower developmental potential. Additionally, the idea of freezing human embryos may present ethical and philosophical dilemmas, as it is something entirely out of the realm of human biology. Statistical analysis confirmed the above, as around 50% of frozen embryos either did not survive thawing or had lower implantation rates.

A major leap

The wide application of the vitrification technique in oocyte and embryo cryopreservation over the last 10 years had a major impact on post thaw survival. While with the older method of slow freezing, the temperature of embryos was gradually reduced to -196° C, during vitrification the embryo reaches the final temperature almost instantly. Ultra-rapid freezing prevents the creation of ice crystals within the cells, which is the main reason why a large number of frozen embryos did not survive. With the vitrification technique, the survival rates have reached 95%, whether applied on oocytes or embryos.

Reliable preservation of embryos at any stage of development opens up new possibilities in the field of human reproduction. Through vitrification, the ovarian stimulation and fertilization process may be detached from the embryo transfer procedure. Put simply, we can stop time for embryos whenever we want and thaw them when the implantation conditions are ideal.

In the case of preimplantation genetic diagnosis (PGD), vitrification allows cryopreservation of embryos during the blastocyst stage, after embryo biopsy, so there is ample time to perform the genetic analysis.

Another major breakthrough was oocyte vitrification. Cryopreserving oocytes reliably has given women the option to preserve their fertility, storing their oocytes while still young and using them at a later time.

Recent studies that have been based on a growing number of births are quite reassuring regarding the safety of vitrification. When comparing children born from vitrified embryos or oocytes to children born from “fresh” embryos, no statistical difference has been found in any significant parameter. As a matter of fact, after examining certain risk factors, such as preterm births and low birth weight, babies born from cryopreserved embryos seem to fair better than the rest!