A revolutionary advance
After an assisted reproduction technique is carried out, embryos that have not been transferred at this time may be preserved thanks to cryopreservation. This has resulted in great progress over the years because, on the one hand, it is possible to not waste the remaining embryos f rom a treatment and, on the other hand, to schedule f rozen embryo transfers in the cases of women who, for medical reasons, cannot undergo a fresh embryo transfer.
In what cases can a fresh transfer not be done?
Usually, at the end of the assisted reproduction cycle, embryo transfer is carried out to complete the process. However, in some cases, the woman’s uterus may be affected by the risk of ovarian hyperstimulation syndrome or the endometrium may not have thick-ened enough and is not receptive. Thus, it is advisable to wait for it to be properly prepared before carrying out the embryo transfer and ensuring the possibility of pregnancy.
What does the technique used to freeze the embryos consist of?
cause the formation of crystals and cell damage. The devitrification, done before the embryo transfer, consists of the reverse process, i.e. a sudden changein temperature from the -196 ºC of liquid nitrogent to the 37 ºC required for the embryo to survive and the gradual elimination of the cryoprotectors used during freezing.The technique that has been consolidated and that is used across the board in all centres is vitrification. Vitrification consists of an ultra-rapid freezing of the embryos after the use of various cryoprotrectors, i.e. substances responsible for protecting the cells from the high speed of freezing. Without them, it would cause the formation of crystals and cell damage.
The devitrification, done before the embryo transfer, consists of the reverse process, i.e. a sudden change in temperature f rom the -196 ºC of liquid nitrogent to the 37 ºC required for the embryo to survive and the gradual elimination of the cryoprotectors used during freezing.
How long can an embryo remain frozen? Does freezing influence its characteristics?
Once the embryos are frozen, they may be preserved for an indefinite time in liquid nitrogen banks that are kept and a very low (-196 °C) temperature and are continually maintained. Thus, the transfer of frozen embryos may be carried out after months, and even years, without losing the quality and characteristics they had when they were frozen.
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In fact, recently – February 2020 – a healthy girl from an embryo frozen for 27 years was born; this is the maximum time observed to date.
What are the survival and transfer success rates of frozen embryos?
As in any assisted reproduction cycle, the likelihood of pregnancy in a cycle with frozen embryos depends greatly on the quality of the embryos and the preparation of the uterus, as well as on various factors involved in the implantation of the embryo. In any case, the process of embryo thawing or devitrification has a survival rate that currently exceeds 95%, and the pregnancy rate using the transfer of frozen embryos does not vary greatly from those using fresh embryos.
This procedure has become a basic tool for human assisted reproduction and, thanks to the high survival rates observed, it has been possible to improve the techniques for achieving pregnancies with a lower number of hormonal stimulations, thus reducing the impact on patients as well.