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Frozen Embryo Transfer

When excess embryos created in the process of IVF are not transferred to the uterus, they can be frozen for future cycles. As eSET (elective single embryo transfer) becomes more prevalent, as is seen in a current worldwide trend to try to prevent birth of multiples as a result of IVF, frozen embryo transfer cycles will increase in popularity. The pregnancy rate following the fresh transfer of a single embryo followed by, if necessary, the transfer of a single thawed embryo is remarkably similar to that of transferring two fresh embryos, while eliminating the risk of twins almost completely.

The number of embryos transferred will depend on the laws of the country where your clinic is situated and/or the Doctor's recommendation , personal preference and quality of the embryos.

In a Frozen Embryo Transfer cycle, the uterine lining may be prepared using hormones or the transfer may be performed using a natural cycle.

The embryo(s) is/are thawed and transferred to the uterus in the same process as in a fresh IVF cycle.

It is said that frozen embryos are less successful than fresh embryos, however some women who have been successful with frozen embryos feel that some factors actually make it more likely to succeed:

  • The preparation stage, which is emotionally draining, is much simpler
  • It is easier to control the thickness of the uterine lining
  • There is no egg retrieval process - another high-stress point
Embryo Freezing
As simple as it may sound, embryo freezing is a complex process. There are many techniques in use today for embryo freezing. There are slow methods for freezing and fast methods. Embryos can be frozen immediately after fertilization (pronuclear stage freezing), while the embryos is in the early stages of development (cleavage stage freezing) or in the later stages of development (blastocyst stage freezing). It is unclear if any one method is better than another.

Slow embryo freezing method

The traditional method of freezing embryos is the slow freeze method. These techniques lower the temperature of the embryos gradually. One of the dangers of embryo freezing is the formation of ice crystals. If ice crystals are present when thawing the embryos, they can lead to damage of the embryo and the embryo may not survive the thawing process. In order to reduce the chance for ice crystal formation, before the embryo is frozen, it is soaked in a solution known as "cryoprotectant".

Cryoprotectant is a solution used to protect biological tissues from freezing damage. The slow freeze method of freezing embryos relies on low initial concentrations of cryoprotectant. This low concentration of cryoprotectant is introduced when the embryo is still at a relatively warm temperature and is still experiencing the normal functions of living cells. The cryoprotectant will permeate and fill the embryo, replacing most of the water in the embryo.

The embryo is then inserted into a small plastic container and placed into a cooling chamber. By injecting tiny amounts of liquid nitrogen into the cooling chamber, the temperature is then lowered slowly to around -6°C. A computer controls the cooling process. The temperature is further lowered to around -32°C. By now, the metabolic rate of the embryo is quite slow. The embryo container is then plunged into liquid nitrogen, causing the temperature to drop even further which completes the freezing of the embryo.

Thawing of these embryos requires a fast approach. The embryos are rapidly warmed to room temperature to prevent ice crystal damage.

Rapid embryo freezing method (vitrification)

A newer method of freezing embryos is the rapid freeze method. The technical termis vitrification. To prepare an embryo for rapid freezing, the concentration of the cryoprotectant is much higher than that used for slow freezing. Again, this is accomplished while the cells of the embryo are at room temperature. The embryo cannot stay at this temperature for long and is immediately plunged directly into the liquid nitrogen which will immediately drop the temperature by a large amount.

An extremely rapid rate of thawing is used once again to prevent ice-crystal damage.

Vitrification is a newer method for freezing embryos. The possible advantages of vitrification are a lower chance for ice crystal formation and therefore a higher chance for survival of the embryo when it is thawed. Some studies have even suggested the possibility that embryos could be frozen and thawed repeatedly without being damaged.

Frozen embryo transfer success rate

Do frozen embryos work as well as fresh embryos?This is a question that is often misunderstood even by doctors who are supposed to be experts in fertility.
One of the main factors in determining the chance for pregnancy in IVF is the number of embryos transferred. If there are less embryos transferred, this can lower the chance for pregnancy.Usually ufter defreezing number of embryos are less in comparison with fresh cycle.In this case, the chance for pregnancy on the frozen cycle would be lower than the fresh cycle. Not because the embryos were frozen but because only less  embryos were available on the frozen attempt.  Second, there are some programs that will freeze any extra embryos, regardless of the embryo quality. Of course, when these embryos are thawed, there is a lower chance for pregnancy not because they were frozen but because they were lower quality to start with.

Finally, of course, some programs simply have difficulty with embryo freezing and have lower success rates across the board.

 There have been several studies in which the rate of pregnancy between fresh and frozen embryos has been compared. In some of these studies, when the number and quality of the embryos is taken into account, it has been found that there was no difference in the pregnancy rates.

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