Following IVF treatment, there may be more embryos created than can reasonably be transferred to the womens uterus. In approximately 50% of IVF cycles the remaining embryos are of sufficient quality to be frozen and stored for future use, giving couples the opportunity to have an additional embryo transfer without the expense of a full IVF cycle.
During a fresh stimulated IVF cycle a certain number of embryos are transferred to the client and the remaining embryos are assessed to determine whether they will be frozen for later use. It is important to note that not all developing embryos are suitable for freezing. Embryos can be frozen at any stage between day 1 (2PN stage) and day 6 (blastocyst stage) after egg retrieval, however, freezng only embryos that survive to the blastocyst stage maximises the chance for success in the thaw cycle. Only high quality embryos are cryopreserved because they are far more likely than others to survive and be capable of further development after thawing. Depending on the time of freezing, 60 – 90 % survive the freeze/thaw process resulting in a pregnancy.
The embryo freezing process takes approximately 3 hours. Embryos are sequentially treated with varying concentrations of a special solution, called 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 client. The straws containing the embryos are placed into a freezing machine, slowly cooled to -35degrees centigrade and stored in liquid nitrogen for long-term storage (-190C) . Considerable care is taken during the process to minimize any possiblity of damage to the embryos. Embryos can be stored for a period not exceeding 10 years.
Embryo thawing is the reverse of the freezing process and involves the warming of them. Our embryologists 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, they 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 transfer, during which time they should resume development and undergo cell division.
Not all embryos survive thawing and not all of thos embryos that do survive will initiate division. The embryos that do not start to grow again have a much reduced chance of implantation.
If there are greater than 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 embyros will re-initiate division and continue to develop in culture whilst others will fail to grow and die. The additional 2-3 days of growth increases the chances of identifying the embryos for transfer that have a high potential to implant. Research has suggested that this methon can enable a pregnancy to be extablished more quickly(i.e. after fewer cycles) than using the “thawing and immediate transfer” strategy.
More embryos may develop successfully than the client has requested for transfer and they may be re-frozen fi they are of suitable quality. However, this does not happen often.
Unfortunately sometimes no embryos develop to the appropriate stage and are therefore unsuitable for transfer. The cycle will be cancelled at this time.
As no one can predict how many thawed embryos will survive intitial thawing, nor those that will/will not develop successfully over the 2 -3 days, it is not possibel 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 our IVF Doctor and /or Consultant Nurse.