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The embryo freezing process takes approximately 3 hours. Embryos are sequentially treated with varying concentrations of a special solution, called the cryoprotectant. The cryoprotectant protects the embryos during the freezing process. Each embryo is then drawn into a specially designed sterile straw. The straws containing the embryos are placed into a freezing machine, slowly cooled to -35°C and stored in liquid nitrogen for long-term storage (-196°C).
Why freeze embryos?
If there are enough embryos of sufficient quality remaining after embryo transfer, these can be frozen or cryopreserved for future use. Using stored embryos for future treatment avoids the need for stimulation, egg collection and fertilisation, and makes maximum use of these very precious resources.
How are embryos frozen?
The cryopreservation process is complicated and takes 2-3 hours, during which time the embryos are carefully placed in a series of special protective solutions before transferring to small sealed plastic straws and frozen slowly using a computer-controlled freezing machine. This is followed by storage in large tanks of liquid nitrogen at the extremely cold temperature of minus 196 degrees. All tanks are fitted with alarms which notify us remotely 24 hours a day of any change in temperature which may require our attention.
Before embryos are stored we ask you to make some decisions about the storage ? how long you wish to have them stored, what to do with them in the event of divorce, separation, incapacitating illness or death. These are all important decisions for you and your partner to consider, and we will contact you each year that your embryos remain stored to check that your wishes remain the same. If your circumstances have changed or you change your mind, you can contact us at any time to vary the conditions of storage.
Will my embryos be frozen?
Only the best grade embryos have the potential to withstand the freezing process which is somewhat stressful to the cells, and we expect an average 70% of embryos to survive freezing and thawing. Occasionally none of a patient?s embryos survive and in order to minimise this risk we recommend that no fewer than 3 good grade embryos are frozen. If there are fewer than 3 surplus embryos of good quality we will keep them in the incubator and can freeze them if they reach the blastocyst stage.
Embryo freezing should be regarded as a bonus - only about a third of couples will have embryos frozen in any one treatment cycle. There is no charge for the initial freezing of embryos and the first year of storage. Subsequently there is an annual storage fee (currently £125), payable in advance.
How do I use my frozen embryos?
Frozen embryos can be transferred in two types of treatment cycle: if you ovulate reliably embryos can be replaced in your natural cycle, otherwise you will need an ?artificial? cycle using drug therapy to prepare the endometrium to receive the embryos. Embryos will be carefully thawed at a time appropriate to their cell number, and the embryo transfer will be scheduled very carefully so that embryo stage and uterine receptivity coincide to ensure the optimum chance of success. The embryo transfer will be performed in the same manner as for your fresh embryos.
How successful is freezing embryos?
Pregnancy rates for frozen embryos transfers are significantly lower than for fresh embryo transfers but depend on the number and quality of embryos frozen. Typically they are between 10-20% per embryo transfer depending on the number, stage and survival of embryos frozen.
Are there any risks with freezing and thawing embryos?
As already mentioned, the greatest risk with freezing and thawing embryos is damage caused by the process itself, despite the care we take to minimise this. Not all embryos are able to withstand the stresses of the necessary dehydration for freezing and rehydration during thawing, hence a reduced survival rate and subsequent failure to resume division and growth for some embryos. This also accounts for the lower pregnancy rate following transfer of frozen-thawed embryos. In a very few cases no embryos survive, or they may survive but all stop developing early. This means for these patients no embryo transfer takes place.
To date there is no conclusive evidence that freezing and thawing embryos causes long-term damage to them, but as with all assisted conception procedures the technology is relatively new and there have been no really long-term studies carried out. To the best of our current knowledge the techniques employed are safe and not harmful in any way.