Freezing allows the preservation of embryos that have not been transferred during the current attempt. Untransferred embryos may, with your consent, be frozen if their appearance permits, but they must have morphological characteristics attesting to their solid constitution, that is to say their ability to resist the process of “freezing”: In practice, some embryos have small fragments or the cells that compose them are very irregular.
These embryos are unlikely to resist freezing, especially if the degree of fragmentation is important. It is therefore not surprising that all the remaining embryos, called “supernumerary”, cannot be frozen.
If the couple accepts this medical hibernation, embryos that are not transferred to the uterus can be stored in liquid nitrogen at minus 196 degrees Celsius.
When the couple so decides, within a five-year legislated period, these embryos can be “thawed” or “woken up” from their biological sleep and then deposited in the uterus. Since the birth of Zoe in 1984 in Australia, hundreds of thousands of children born from frozen embryos have been born around the world. If you refuse to freeze, you must notify the laboratory before the IVF attempt.
In this case, it will only be necessary to fertilize a limited number of oocytes (corresponding to the maximum number of embryos transferred).
How is an embryo frozen?
To be frozen, the embryo formed largely of water undergoes a preparation or the water is replaced by a cryoprotectant, “biological antifreeze”. This prevents the formation of ice crystals that would damage the embryo.
The embryo is immersed in successive baths containing the cryoprotectant and sucrose at different concentrations. Sucrose makes the medium hypertonic with respect to the embryo, resulting in the release of water and the return of the cryoprotectant.
The embryo is placed in a plastic straw 13 cm long and 2 mm in diameter containing a volume of 0.3 ml, a part of the straw contains all the indications necessary for a good identification.
Finally, the medical technicians gradually lower their temperature, down to -196 degrees Celsius, in an automatic device, and according to very strict criteria. They are stored at this temperature in liquid nitrogen.
How is the embryo defrosted?
The straw containing the embryo is released from the liquid nitrogen. Therefore, it is necessary to rehydrate the embryo by successive baths containing less and less cryoprotectant and more and more aqueous medium, at a temperature of above 37 degrees Celsius.
The embryos that survived the freeze-thaw process are transferred the same day of thawing. Storage in liquid nitrogen can last for years without alteration of the embryos.
What happens to these frozen embryos?
If there is no pregnancy after IVF, frozen embryos are transferred in subsequent cycles. If pregnancy occurred during IVF, the embryos are kept for a maximum of 5 years. The couple is consulted each year in writing about their desire regarding the future of frozen embryos.
In all cases, whether or not there has been a pregnancy, there will not be a new attempt at IVF as long as there are still frozen embryos.
Frozen embryos can only be transferred in the presence of both spouses. In case of separation of the couple (divorce, death) the frozen embryos cannot be transferred. You will receive a mail every year asking you what you want to do embryos: either use them for a new transfer. See them for one more year and the embryo conservation application must be renewed in writing each year.
If you do not want other children, the future of frozen embryos will be regulated according to the bioethics law in force: currently, the choice is given between the stop of conservation , and the gift for another couple or for the medical research