During IVF or ICSI procedures where only one embryo is transferred, good quality embryos can be frozen (the medical term is ‘cryostorage’). Thawed frozen embryos can then be used at a later date if treatment is unsuccessful or if you wish to have further children. Embryo freezing is also a treatment option if you have a medical condition that is likely to affect your fertility or if you have not been able to have an embryo transfer after IVF or ICSI.
To understand more about how frozen embryos are used, visit our frozen embryo transfer page.
When are embryos frozen?
Embryos can be stored at any stage from day 1 to day 6.
Embryos are rarely frozen on day 4, because it’s difficult to understand their survival when thawed or warmed.
Embryos are only frozen after a couple have signed a form consenting to the storage of their embryos. This consent form will also ask the couple to decide on the disposal of any unwanted or unused embryos.
Why are only some embryos frozen?
Not all embryos develop well in the laboratory. Any embryos which are not well developed (frozen or not frozen) have little chance of resulting in a pregnancy so freezing all embryos can give couples false expectations and increased costs.
How can you tell if an embryo has developed?
Embryos that have cells of equal size and shape, with little or no fragmentation have the greatest pregnancy rate. Therefore if an embryo has developed to the 6‐cell stage or greater by day 3 of culture can be frozen, providing they have little or no fragmentation. All embryos not suitable for freezing at day 3 will be cultured for a further three days and any good quality blastocysts will be vitrified and cryostored. Sometimes all embryos are grown to day 5 and cryostored at the blastocyst stage. The decision to cryostore at different stages will be discussed with you and your partner by your doctor.
How are embryos frozen?
The medical term for frozen is ‘cryostorage’, where we use a technique of cooling embryos called “vitrification”. Vitrification refers to the “glass like state” of the cryopreserved (frozen) embryo. Vitrification involves changing the temperature of the embryos very rapidly and uses a higher concentration of “cryoprotectants” than is used in slow freezing. Cryoprotectants are chemicals added to the solution around the embryo to protect it from damage from the formation of ice crystals.
How long can an embryo remain in cryostorage?
Embryos can be stored indefinitely. Australian regulatory authorities allow a maximum period of five years for embryo storage. Storage for greater than five years requires that a couple complete a written application to prolong the storage for a further period of time (usually up to ten years).