Cryopreservation, or embryo freezing, is an essential part of courses of assisted reproduction treatment since it means that embryos can be preserved in order to be used at a later date and without the passing of time having a negative impact on their viability.
Embryos obtained as a result of in vitro fertilisation (IVF) may be thawed for a number of reasons:
- When an excess number of embryos are available following fresh transfers and the couple has not become pregnant following the initial procedure.
- When couples who wish to opt for a second pregnancy following a successful first pregnancy have surplus embryos.
- When fresh embryo transfer has not been possible for whatever reason (ovarian hyperstimulation syndrome, inadequate endometrium, etc.), and the total number of embryos obtained have been preserved.
Embryo cryopreservation is a consolidated technique for embryos at any stage ranging from the pronucleus (zygote) to the blastocyst (day 5 or 6 of development). Frequently in such cases, there are not a great number of remaining embryos which can be frozen since not all embryos reach such an advanced stage of development (the blastocyst formation rate is 60%). Additionally, freezing on day 4 of development is increasingly carried out.
Lysis of some or all of the embryo cells can occur during freezing and thawing, Embryos in which at least 50% of the cells survive are considered suitable for transfer.
The success of the technique will depend on the embryo’s capacity to survive the cryopreservation process. The average survival rate is 98% and successful pregnancy rates are around 40%.
It’s also important to point out that survival rates in frozen embryos which have previously undergone a biopsy do not alter.
Therefore, thanks to progress in vitrification techniques, the possibilities of getting pregnant using frozen embryos are currently similar to those obtained in fresh transfer treatments.