Embryo Freezing – Cryopreservation Options for Couples
During the in vitro fertilization process, there are often more excellent quality embryos that develop than can be transferred into the woman’s uterus at the time of her fresh transfer. In these situations, we have the ability to offer our patients the opportunity to freeze these “extra” excellent quality embryos. Embryo freezing, sometimes called “cryopreservation” or “vitrification” allows patients fortunate enough to have excess embryos to have another chance for pregnancy without having to go through another ovarian stimulation and oocyte retrieval, and without having to buy more expensive stimulation medications and take shots.
Among those embryos that continue to divide, there are some that meet specific criteria that allow them to be successfully frozen. These embryos are usually frozen (vitrified) on day 5 or on day 6 after the eggs have been retrieved. Vitrification of these excess embryos provides an opportunity for achieving pregnancy at another time in the future, thus increasing the number of potential transfers from a single retrieval.
There are several methods that have been used for freezing embryos.
They can be divided into two categories; the slow-freezing process and the vitrification (quick freeze) process.
Embryo Freezing with Slow Freeze
During the slow-freeze process, embryos are exposed to cryoprotectant solutions (an antifreeze product) for a longer period of time while the temperature of the embryo is reduced at a specified rate until it is fully frozen. This is an older method of cryopreserving embryos, Texas Fertility Center and Austin IVF no longer use this process.
Embryo Freezing with Vitrification
With vitrification, embryos are exposed to a higher concentration of cryoprotectant solutions for a very short time and are then frozen very quickly. The general principle behind both processes is very similar. Basically, an embryo is exposed to a cryoprotectant freezing solution in order to remove all of the water from the cells. While this is being done, the temperature of the embryo is lowered, allowing it to be completely frozen.
The process of vitrification has been used successfully for several years at Austin IVF, after perfecting it in animal embryos. It allows the embryos to be exposed to cryoprotectant solutions for a much shorter time before being completely frozen. This short exposure time and rapid freeze has improved embryo survival and pregnancy rates dramatically in many clinics over the last few years.
We began using vitrification for cryopreserving embryos in 2010 and we have noted remarkable changes in both survival rates and in pregnancy rates compared to the use of the slow-freeze techniques. Embryo survival rates have increased from 70-75% with the slow-freeze process to 95-97% with the vitrification process. Most importantly, delivery rates are now approaching those rates seen during fresh cycles. If this continues, electively freezing all of a patient’s embryos for a later frozen embryo transfer may become a viable alternative to fresh embryo transfer, as it may eliminate the risks of ovarian hyperstimulation syndrome and high order multiple pregnancy.
Austin IVF remains on the forefront of perfecting the protocols involved in embryo vitrification and the results that we are seeing in our laboratory are rapidly changing our approach to the IVF patients in our fertility practice at Texas Fertility Center.
Frozen Embryo Storage
All frozen embryos are stored in liquid nitrogen, which has a temperature of -196 degrees C (-320 degrees F). Storage at this extremely cold temperature halts all metabolism of the embryo, allowing embryos to be safely stored for an indefinite period of time.
Once a woman is ready to proceed with frozen embryo transfer, we give her estrogen pills and progesterone in order to prepare her uterine lining for pregnancy. On a very specific day – that depends on the cell stage at which her embryos were frozen – her embryos are thawed. During this process, the embryos are exposed to decreasing amounts of cryoprotectant, eventually clearing the cryoprotectant completely from the cytoplasm of the embryo.
Embryos are then assessed closely to make sure that they survived both the freezing and thawing processes intact. Surviving embryos are placed into culture media and incubated until transfer. This incubation period allows the embryo to complete its recovery and begin to resume its metabolism.