Cryopreservation or freezing of embryos by vitrification
What is cryopreservation?
During an In Vitro Fertilization from eggs obtained following an ovarian stimulation and follicular puncture, it is common to obtain several good quality embryos on day 5 and/or 6 of the embryonic development. One or two embryos, are usually selected for a fresh transfer into the maternal uterus (in Quebec, the transfer of 2 embryos is only authorized for patients aged 37 and over). Good quality supernumerary (surplus) embryos can be preserved (cryopreserved) by a rapid freezing technique called vitrification.
The beginnings of cryopreservation
Although the concept of vitrification has been established since 1937, its clinical practice in human In Vitro Fertilization was not really implemented until the 2000s. The process of direct vitrification of gametes and embryos in liquid nitrogen is fast and allows a thawing survival rate close to 90%, compared to the slow freezing technique.
The stages of vitrification of embryos
Vitrification of the embryo is carried out in two sequences: the first step consists of dehydration of the embryo, by removing most of the water from the cells and then replacing it with a cryopreservative. Thus, the risk of lysis of embryonic cells by the formation of ice crystals is considerably reduced, unlike slow freezing where water is always present in larger quantities.
Once these steps have been completed, the embryos are then immersed and stored in liquid nitrogen at -196 ° Celsius. There is no specific time or deadline to keep or use an embryo, since the storage time does not affect the quality of its cells.
Subsequent use of frozen embryos
These frozen embryos can be subsequently used for frozen embryo transfer (TEC), in a natural or simulated ovulatory cycle, without the need for another egg retrieval. These supernumerary embryos are mainly used if the first uterine transfer of a fresh embryo has not worked and it could also be used for a second pregnancy.
Most of the time, vitrified embryos are frozen the day before a uterine transfer, in order to ensure their survival and growth overnight.
With such embryos, the chances of pregnancy are as good or even better than with a fresh transfer.
To be noted
Since the vitrification technique is used in almost all fertility centres around the world, it is common to use the word frozen to denote vitrified embryos.