embryo freezing / thawingDuring a fresh stimulated IVF cycle 1 or 2 embryos are transferred to the patient and the remaining embryos are assessed to determine whether they will be frozen for later use. Embryos may be frozen between 1 and 7 days after egg collection.
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 that is uniquely labelled for the patient. The straws containing the embryos are placed into a freezing machine, slowly cooled to -35oC and stored in liquid nitrogen for long-term storage (-196oC).
Embryo Thawing
Embryo thawing is the reverse of the freezing process, and involves warming the embryos. Scientists will usually thaw the embryos the day before the scheduled embryo transfer. Embryo thawing takes approximately 2 hours. Once the embryos are returned to room temperature, the embryos are passed through a series of solutions to remove the cryoprotectant that is no longer needed. The thawed embryos are kept in the incubator until the embryo transfer, during which time they should resume development and undergo cell division.
Number of embryos to thaw
Not all embryos survive thawing and not all of those embryos that do survive will initiate division. The embryos that do not start to grow again have a much reduced chance of implantation.
If your embryos in storage were younger than 5 days when frozen then you have several alternatives for thawing them.
This fact sheet is for those embryos which will be cultured for up to 3 days before transfer.
Extended culture
If you have greater then 3 embryos in storage then we would suggest that at least 3 are successfully thawed and cultured for 2-3 days before assessment for transfer.
Once thawed, some embryos will re-initiate division and continue to develop in culture whilst some embryos will fail to grow and die. The additional 2-3 days of growth increases the chances of identifying and selecting the embryos for transfer that have a high potential to implant. In addition, published data suggests that this method can enable a pregnancy to be established more quickly (i.e. after fewer cycles) than using the 'thawing and immediate transfer' strategy.
More embryos may develop successfully than the patient has requested for transfer and are refrozen if they are of suitable quality. However this does not happen often.
Unfortunately sometimes no embryos develop to the appropriate stage and are therefore not suitable for transfer. The cycle will be cancelled at this time.
As one cannot predict how many thawed embryos will survive initial thawing, nor those that will/will not develop successfully over the next 2-3 days, it is not possible to accurately predict the number of embryos that will be used or the number (if any) that will eventually be transferred.
Therefore the number of embryos recommended for thawing is based on the total number in storage and the number requested for transfer. This decision should be made in consultation with your IVF doctor or nurse co-ordinator.
Patients are informed of the thaw results prior to admission. Your patient coordinator will make every effort to contact you in the event that all your embryos have failed to survive the thaw.
Frozen Embryo Transfer (FET)
The timing of the embryo transfer depends on a number of factors including the cell stage of the embryo when frozen and uterine receptivity. It is essential that a Frozen Cycle consent form be completed and returned to the clinic prior to your first blood test or ultrasound scan or prior to commencement of Clearplan.
If the consent form is not completed the scientist will not be able to thaw the embryos.
The overall success rates associated with transferring frozen-thawed embryos are less when compared to a fresh embryo cycle but are still substantial.
If you would like to commence a thaw cycle or have any questions or concerns about frozen embryo cycles, please do not hesitate to contact your Monash IVF patient coordinator.
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