If more than two quality embryos are obtained in an in vitro fertilisation cycle, it is recommended to use vitrification. Quality embryos are stored indefinitely until the woman wants to undergo another cycle or if she wants, they can be donated to other couples.
It is necessary to perform embryo thawing in order to use the cryopreserved embryos.
What is thawing?
Thawing is a process which consists in increasing the temperature from -196ºC to 37ºC, the interior body temperature.
There are several reasons for vitrifying embryos such as a higher number of quality embryos obtained than those which can be transferred, transfer must be cancelled because of an inadequate endometrial preparation or for other causes.
Embryos can remain vitrified for an indefinite period without affecting its quality, legally until the woman’s reproductive age ends or the couple decides to renounce or donate them to other couples.
It is necessary to follow a meticulous process and a specialist to perform the thaw process, as in the case of vitrification.
Embryos are incubated in a series of media and specific conditions in order to perform embryo thawing. Embryos are removed from a medium with a high concentration of cryoprotectants (substances used for freezing) and plunged to other media which are more progressively diluted.
This ensures cell rehydration, so that cryoprotectants are replaced with water. It is very important to respect times and to perform thawing correctly, otherwise embryo cell structures may be damaged.
Embryos are usually thawed on the day of embryo transfer, it must be performed some hours before transfer to check the state of the embryo.
Zona pellucida is the most harmed area during embryo thawing, since it is the most external membrane which has direct contact, its main function is to act as a barrier between embryo and the medium surrounding it.
Vitrification and thawing techniques as well as cryoprotectant substances are more and more efficient and offer better guarantees. These techniques are very usual in assisted reproduction centres, since cryobiology was established many pregnancies have been achieved.