Embryo Freezing and Cryopreservation

Very often in IVF cycles, the number of embryos produced exceeds the amount that will be transferred. There are several options for what to “do” with extra embryos. One option is to freeze the extra embryos for a couple to use at a later time. In this way, if a couple fails to achieve pregnancy on an IVF attempt, they can try again using the embryos that were frozen. This is less costly and less invasive than a fresh IVF cycle. Alternatively, if a couple does achieve pregnancy and delivers a healthy live born baby, they can use their frozen embryos to have additional children later on.

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.

Embryo freezing
Embryo Freezing

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 term is 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.

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. At IVF1, there is little if any difference in the chance for pregnancy whether fresh or frozen embryos are used. Why, then, do other programs say the opposite?

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. Let’s take the example of a couple that has four high quality blastocysts produced in a fresh IVF cycle. Two embryos are transferred and two are frozen. Let’s say the couple delivers a baby from the fresh IVF cycle and then returns to use the frozen embryos a few years later. Upon thawing, however, only one of the embryos survives and it is transferred.

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 one embryo was available on the frozen attempt instead of the two which were used on the fresh 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.

Follow this link to learn more about the process of using frozen embryos to attempt pregnancy which is called a frozen embryo transfer or FET cycle.