For many years, all embryos were frozen using what is known as a slow-cooling protocol. The embryos were placed into a specific solution containing cryoprotectants and the temperature was slowly dropped (–0.33 C/min) in a very precise manner. Slow-cooling worked best for cleavage stage embryos or embryos frozen on the day after egg collection (2 pn stage). Slow-cooling was not as effective when freezing unfertilized eggs or embryos at the blastocyst stage of development.
Vitrification literally means “turning to glass” and is an ultra-rapid form of cryopreservation (–20,000 C/min) that basically consists of plunging the egg/embryo into liquid nitrogen resulting in near instantaneous freezing of the egg/embryo. Recovery rates and pregnancy rates have been shown to be superior when vitrification is compared with slow-cooling of eggs and blastocyst stage embryos. Not all clinics currently offer vitrification, but it seems likely that this approach will become the standard method of cryopreservation over the next 5 years.
Since adopting vitrification in April 2007 at Dominion Fertility, our frozen-thaw embryo transfer pregnancy rates are now equivalent to our fresh embryo transfer pregnancy rates. We have abandoned the slow-cooling approach and use vitrification exclusively for our embryo cryopreservation program. Furthermore, many studies have demonstrated remarkable success in the area of egg (oocyte) cryopreservation. Success with freezing unfertilized eggs may offer new hope to cancer patients and to women considering freezing their eggs for future use. In addition, oocyte cryopreservation may allow for the development of oocyte banks similar to sperm banks.