Embryo thawing: everything you need to know about devitrification

The ability to thaw embryos in a simple and safe way that delivers favourable results has led to significant optimisation of assisted reproduction treatment. It has now become possible thanks to implementation of the devitrification process at in vitro fertilisation (IVF) laboratories.

What is embryo thawing?

Embryo thawing is the process for recovering the physiological temperature of embryos and replacing the cryoprotectant molecules (that enabled preservation at -196ºC) with intracellular water. This reactivates biological activity.

What embryo thawing techniques exist?

The embryo thawing technique itself depends on the freezing process that was used. There are two cryopreservation techniques: slow freezing and vitrification. In the former, ice crystals are formed in a controlled manner using low concentrations of cryoprotectants and a gradual decrease in the temperature. Therefore, during thawing, the physiological temperature is gradually and slowly reached once again and this is followed by replacement of the cryoprotectants with water.

Vitrification, on the other hand, consists of avoiding formation of the aforementioned ice by increasing the concentration of cryoprotectants and the cooling speed. Therefore, a quick heating process takes place during devitrification (from -196ºC to 37ºC in under 1 second) followed by replacement of cryoprotectants with water, which is a must.

Does the embryo survival rate and prognosis for pregnancy alter depending on the thawing technique used?

Although the two aforementioned techniques exist, the excellent results that have been obtained in vitrification/devitrification mean that it has become the technique of choice in IVF laboratories. The survival rate is close to 100%.

The probability of achieving a pregnancy, on the other hand, does not depend so greatly on the cryopreservation technique, but rather on the quality of the embryos and the condition of the endometrium. However, elevated survival rates following devitrification do ensure an improved prognosis in the long term since it increases the number of available embryos.

How long does the embryo thawing process take?

The embryo devitrification process is a relatively quick one. It takes around 15 minutes. However, it should be taken into account that some of the tools required to perform the procedure need to be at 37ºC and so prior preparation is necessary.

How much time can go by between embryo devitrification and transfer to the uterus?

The time that goes by between embryo devitrification and transfer depends on the day of development on which they were vitrified and the conditions under which transfer needs to take place.

• If they are vitrified on Day +3 and transfer needs to take place on Day +3, devitrification takes place on the same day as transfer.
• If they are vitrified in blastocyst stage, devitrification takes place on the same day as transfer.
• If they are vitrified on Day +3 and transfer needs to take place on Day +5, devitrification needs to take place prior to transfer and the embryos are left to continue developing.

Whatever the case, it is important to devitrify embryos a few hours before cryotransfer so that it can be confirmed that they have survived the process appropriately.

When are embryos that have been frozen and thawed implanted?

Implantation is the process of putting the embryo and the endometrium into contact so that gestation can begin. This union takes place between 6 and 7 days following fertilisation when the embryo is in blastocyst stage. It makes no difference if the embryo has been previously cryopreserved or not.

Do the chances of a successful pregnancy change if the embryo is fresh or cryopreserved?

It should be kept in mind that the chance of pregnancy by means of IVF depends on numerous factors and personalising treatment is of utmost importance.

The results that were formerly obtained with cryopreserved embryos were slightly lower than those obtained when fresh embryos are used. However, optimisation of cryopreservation techniques has meant that the results are now very similar.

Furthermore, in certain cases, the freeze all strategy delivers better results than fresh transfer because the endometrium is as prepared as it possibly can be.

Does the duration of embryo vitrification have any impact?

Vitrification is a relatively new technique and potential effects are still being researched. However, several studies have demonstrated that there is no negative impact in terms of survival and implantation of embryos cryopreserved in liquid nitrogen for years. In fact, the birth of several children following thawing of embryos that had been cryopreserved for over 20 years has been reported.

Is the health of a child born from a frozen embryo affected?

Many factors have an impact on the development of a child and they are difficult to analyse individually (the mother’s age, the mother’s weight, the environment in which the child develops, the reason for sterility, etc.). Available data on the health of children born following embryo thawing are constantly reviewed and analysed. Several studies highlight differences with regards to birth weight between these kinds of embryos and the ones that are transferred when fresh. Other impacts of cryopreservation on the health of babies have not been demonstrated.

Is freezing an embryo from a frozen oocyte (ova) an option?

Yes. Embryo vitrification is possible as long as the embryo reaches the required state in good quality, irrespective of the origin of the gametes that made it. It is true that cryopreservation, just like any other procedure performed in an IVF laboratory, does imply a certain degree of stress for gametes and embryos. As such, the techniques that are used in each case must be individually assessed.

Laura Cascales, biologist at Instituto Bernabeu