Embryo thawing is a process where embryos are warmed from a cooled state to prepare for transfer to a uterus. Multiple embryos are often created as part of an in vitro fertilization (IVF) procedure, but only a certain number of embryos can be transferred to a uterus at one time.
If there are remaining embryos, or there is a situation where a transfer cannot occur, they are likely to be cryopreserved through a process called vitrification and can be used at a later date for a pregnancy attempt.
Because there is no ice formed during the process of vitrification, the process is technically referred to as embryo warming over embryo thawing.
FertilitySmarts explains Embryo Thawing
The process of vitrification essentially freezes an embryo in time by bringing them to a temperature where all cellular activity stops. In order to protect the embryo through this process, cryoprotectants are added and the embryo is dehydrated.
The process of thawing the embryo is the opposite of this freezing process. It is a less precise, but faster process. Since the embryo is dehydrated during freezing, it must slowly and carefully be rehydrated while it is first being warmed to room temperature. The embryo is warmed through a series of water baths and air, along with the removal of all the previously added cryoprotectants.
After it is warmed to room temperature, it is then warmed to a normal body temperature of 98.6 degrees Fahrenheit. Once the embryo is rehydrated, warmed to body temperature, and all the cryoprotectants have been removed, it is normally allowed to rest for a few hours prior to a frozen embryo transfer (FET) procedure.
The timing of thawing of the embryo should be directly related to endometrial receptivity, which is essentially having a thick uterine lining that is receptive to embryo implantation.