Embryos can be frozen at any stage of development during the IVF process. Eggs that are fertilized can freeze as early as 1 day after an egg retrieval procedure, but it is more common to allow embryos to develop for a number of days before freezing them.
In IVF, usually more embryos are generated than needed for one particular embryo transfer. Any excess healthy embryos from the IVF process can be frozen, rather than discarding them into the dustbin. These very frozen embryos can be revived again, process being known as “Thawing”, and used for additional attempts at embryo transfer if needed. Process of embryos freezing, also known as cryopreservation, takes place for some 60% of all patients having IVF treatment; and frozen embryo transfers accounts for around 50% of all IVF births.
Embryo freezing gives more opportunities of pregnancy for each hormone stimulation cycle and egg collection.To avoid serious risks associated with multiple pregnancies, not more than 1-2 embryos are transferred at a time. For example, if more than 2 healthy embryos are obtained (this does not always happen), then usually transferring two embryos are recommended and freezing the remaining ones. If pregnancy is not conceived in the first cycle, then the spare embryos are transferred in subsequent cycles without making it to pass through another cycle of hormone stimulation and egg collection. This process is known as Frozen Embryo Transfer (FET). However, this procedure is taxing for sensitive biological material like embryos and retrieval rate of frozen embryos remains around 50% only. A unique advantage of embryo freezing is that it preserves fertility potential of a woman which is otherwise bound to go down with advancing age. For example, a woman gets her embryos frozen at the age of 30 and utilizes the same at the age of 36; the probability of success in her case will be similar to that of a woman of 30 rather than 36.Once the IVF treatment is finished, there are several options available. The embryos can be stored at IVF facility, where they will be kept frozen in cryostorage until decided to use, donate or discard them. If the frozen embryos are not needed by the subject then it can be donated to another couple who are unable to conceive with their own embryos.
Embryos can be frozen from Day 2 (four cell stage) to Day 5 (Blastocyst). They are placed in thin plastic straws, sealed at both ends, and labeled with the name of the subject and identification number. Then they are stored into a freezing machine, where the temperature rapidly drops to -150° Celsius. The straws are then placed in goblets, and put into tanks filled with liquid nitrogen, which keeps the temperature at -196° Celsius.