When an embryo is ready to be used for transfer it can be thawed out. Although if the embryo has been rapidly frozen using the vitrification procedure, it is locked into a glass-like state without any harmful ice crystals so technically the embryo isn’t thawed… it is ‘warmed’.
Warming embryos is essentially the reverse of cooling (freezing) them. The freezing process draws the water out of the cells so they need to be gradually rehydrated as the embryos recover during thawing. This is done by moving an embryo through solutions designed to draw out the toxic cryoprotectant fluid and replace it with water. The solutions must be prewarmed to 37°C and maintained at the correct pH to ensure the embryo is thawed directly into the optimum environment. This stops the embryo encountering any unnecessary stress which could affect its survival. Precise timings are important during embryo warming so it is important the act quickly but carefully to give the embryo the best possible chance.
If you have more than one embryo cryopreserved, they will likely be thawed in order of their quality with the highest grade embryos being chosen first.
Embryos are actually very resilient and they handle the freeze-thaw process very well. The survival rate after freezing should be around 95% if the embryo is handled correctly. However, immediately after the embryo has been warmed it often looks squashed so it is difficult to tell whether it is still viable. This is because a blastocyst has a large fluid-filled cavity which is emptied before freezing. At this stage you may hear the embryo being described as ‘collapsed’ which sounds worse than it is – it is actually very normal. Many clinics will leave the embryo in an incubator for a few hours or even overnight after warming so it can re-expand fully and therefore be assessed more accurately, which can give a better idea of its potential. Other clinics may thaw the embryo immediately before transfer because they believe that the embryo should be given the opportunity to re-expand inside the uterus which might be a more favourable environment for it to recover in.
It is also possible to refreeze an embryo again after it has been thawed. For example, this might be done if parents want the frozen embryo to be genetically tested. In this case it will be thawed and biopsied, then refrozen while we wait for the results.