Approximately ¼ of the children born today through assisted reproduction techniques come from cryopreserved embryos or ova. The idea of human embryo cryopreservation was conceived by the pioneers of IVF; however, the first ones to successfully achieve a pregnancy from the transfer of a frozen-thawed embryo were Trounson and Mohr in 1983 in Australia, and Zeilmaker and his colleagues in 1984 in the Netherlands.
Freezing embryos maximises the use of embryos produced during a fresh cycle and increases the likelihood of conception from the same group of embryos, while, at the same time, reducing the number of multiple gestations. The method of rapid ova and embryo cryopreservation, also known as vitrification, is a relatively recent method, as opposed to the traditionally used controlled progressive freezing. Conventional freezing methods are not always successful, as they allow for the creation of crystals within the cells, thereby harming the embryo and causing irreversible damage to the ova. The procedure of vitrification consists of initially placing ova and embryos within a small volume of a highly concentrated anti-freeze substance (DMSO and ethylene glycol) for a short period of time before being frozen at a very rapid rate. This rapid freezing rate significantly decreases crystal creation. The water inside the cells automatically freezes and there is no time for crystals to form, which could have potentially caused irreversible damage to the ova and embryos. After vitrification the ova and embryos are stored in liquid nitrogen containers until the moment they are thawed for use. The most recent scientific data show that vitrification can be successfully applied for freezing embryos, at all pre-implantation stages, as well as for freezing ova. Especially concerning freezing blastocysts, vitrification seems to be associated with higher survival and successful pregnancy rates when compared to controlled progressive freezing.
In such cases when we would choose to place embryos in cryopreservation, a special medication regime is implemented and the thickness of the endometrium is checked before the embryo transfer is performed during the cycle we choose for the implantation.
During the freezing - thawing procedure approximately 30% of cells is destroyed because not all embryos have the same capacity to survive such an ‘ordeal’. This is believed to create a protective selection, creating the most normal and resilient embryos. Based on research so far,the health of children originating from cryopreserved embryos or ova is no different that that of children born after IVF.
The transfer of frozen embryos increases the overall pregnancy rates achieved in a woman with only one ovum collection by approximately 10%-15%. Furthermore, freezing embryos seems to be a safe method that does not cause chromosomal anomalies, regardless of the length of cryopreservation.
In conclusion, freezing embryos provides a satisfactory solution to the problem of ‘superfluous’ embryos, while increasing cumulative IVF pregnancy rates.