Parts of the embryos obtained will be transferred to the uterus in a fresh IVF cycle. The rest will be vitrified while waiting for their destination to be decided between the legally possible options: donation to other couples, donation for research, cryopreservation to try for a new pregnancy in another cycle or destruction.
In this article we are going to explain the vitrification process by which embryos are preserved until their future use.
The different sections of this article have been assembled into the following table of contents.
Cryopreservation of embryos
Vitrification is a process that allows the preservation of embryos for an unlimited time without changing their characteristics. This is the ultra-fast freezing of the embryos, which avoids the formation of ice crystals inside the embryo and therefore reduces the possible damage to the internal structure of the embryo as much as possible.
This cryopreservation method allows for better survival and implantation rates since the embryos, after thawing, maintain practically unchanged the characteristics of the moment in which they were vitrified. All this translates into increased pregnancy rates achieved by vitrification compared to slow freezing.
The vitrification process
The embryos can be vitrified on day 2 or day 3, but also on day 4 and even in the blastocyst stage.
Something generalized in surrogacy is the genetic analysis of the embryos, so in case of subjecting the embryos to PGD techniques, they should be vitrified on day 5 or 6, as long as their genetic endowment does not show alterations. The biopsy performed on the embryos does not prevent their vitrification.
Vitrification involves the ultra-fast freezing of the cells, in this case the embryo. The cooling rate in vitrification is approximately 115,000 ºC per minute; an extremely higher rate than slow freezing where approximately 0.3ºC per minute was decreased.
Once vitrified, the embryos are stored in liquid nitrogen banks at minus 196 ºC. At present, there is no evidence that the storage time of embryos has consequences on their viability or may affect the chances of implantation.
Although current technological advances in embryo vitrification techniques allow for very high recovery and survival rates, it is important to vitrify only those embryos that are of sufficient quality to endure the vitrification and devitrification process and to arrive at the moment of transfer with high guarantees of success.
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inviTRA, 2019. What is Embryo Vitrification? Advantages over Slow freezing. Edurne Martínez, Mark P. Trolice, Zaira Salvador and Sandra Fernandez. https://www.invitra.com/en/embryo-vitrification/
inviTRA 2019, What is Assisted Reproductive Technology (ART)? Techniques & Costs. Blanca Paraíso, Sara Salgado, Zaira Salvador and Sandra Fernandez. https://www.invitra.com/en/assisted-reproduction/