Freezing of biological material has been an essential breakthrough for the field of assisted reproduction, as it facilitates multiple attempts at embryo transfers without the need for ovarian stimulation and egg retrieval in every cycle. However, it does have its drawbacks, with the most prominent being the induction of freeze damage to the preserved tissues. Freeze damage is caused due to the physical properties of water molecules contained within embryonic cells, oocytes, and spermatozoa. In temperatures below 0 degrees Celsius (32 degrees Fahrenheit), water turns to ice, forming sharp crystals and expanding, thus causing trauma to surrounding cell components. The effects of this process become apparent during thawing when the cell regains its basic metabolic functions, but by then, the damage may be severe enough that it is unable to repair it and perishes. This is exacerbated in the case of eggs (oocytes), which, unlike embryos, do not grow or proliferate and thus lack effective repair mechanisms.
During egg freezing, Embryologists take precautions to mitigate the effects of freeze damage as much as possible. In particular, before freezing, they replace a large part of the water contained within the oocytes with cryoprotectant agents, non-toxic substances that do not form crystals and thus do not cause trauma. This can drastically improve the odds of survival of the oocytes. However, the most important factor seems to be the freezing methodology. When cryopreservation was first introduced, slow-freezing was the prevalent methodology, whereby oocytes are gradually introduced to lower temperatures, down to the temperature of liquid nitrogen. The new methodology introduced later on, and currently the standard practice, is vitrification, whereby oocytes are rapidly cooled down to the temperature of liquid nitrogen. With this methodology, ice crystals do not have the chance to form, and ice does not expand, thus mitigating freeze damage even more. Once frozen, the oocytes can be preserved in stasis for many years, and if they survive the thawing process can be fertilized and transferred. In general, survival rates are better after vitrification and have been reported to reach up to 84-99%, whereas survival rates after slow freezing are generally lower, with reported values reaching 74-90%. This may vary among different Embryology Labs based on protocol specifics, culture media, storage and freezing devices, etc. Additionally, it should be noted that oocyte survival is only the beginning and that fertilization and embryo development to 3 and/or 5 days are important checkpoints that must also be successfully cleared before a transfer can be attempted. Therefore, it is preferable to cryopreserve embryos rather than oocytes, provided that a suitable partner or sperm donor is available.
Freezing techniques have been improving over the years, with the introduction of vitrification in laboratories we can freeze oocytes knowing that we will have a good survival rate. Oocyte vitrification survival is around 90%. Although this also depends on the quality of the oocyte and the age of the patient, studies confirm that the survival rate of frozen eggs increases in young patients.On the other hand, a good number of frozen oocytes will give the patient more chances to have a good cycle and get pregnant. Egg vitrification is a highly demanded and recommended technique nowadays, being used for the preservation of fertility for social or medical reasons.
Because the egg freezing process has been so successful in recent years, banks like us frequently freeze eggs and thaw eggs and get successful pregnancies as a result. It again goes back to the quality of the eggs but in the past because we couldn’t freeze eggs very successfully, the fresh cycles were always more successful in giving pregnancy than frozen cycles of eggs so if you had egg donation or if you froze your own eggs, the frozen eggs may have not been that successful. Nowadays this is completely changed. We have a very good survival rate of eggs after freezing. We have a very good fertilization rate, blastocyst development and also successful pregnancy; for example we know from our own bank where we provide a good quality donor eggs, we have a 95% survival of our eggs we have a very good number of fertilization around 86% and we have over 75% pregnancy rate with the eggs that we freeze and provide to patients that need it.
The reason for this success is because the eggs are from women that are younger – between the age of 20 to 32 and therefore these are healthy eggs that have been screened and will survive the process very well and produce pregnancy.
If you are freezing your own eggs and would like to know the success rate of it in the future it all goes back to what age you were, how many eggs you’ve frozen, the quality of the eggs you’ve frozen, the technology that’s used in the lab that you freeze eggs.
If you would like to freeze your eggs for the future, it is very important that you go to a clinic that is an expert and has experience in freezing eggs and also thawing or warming the eggs afterwards for the use in an ICSI cycle.
Survival rate of eggs from freezing is usually about 70 %. Some clinics can achieve success rates as high or survival rates as high as 90 % but that’s usually with younger patients.
As we’ve explained before also vitrification has allowed for a very good oocytes survival rate for young patients that’s below 38. Survival rate is around 85-90 so that’s a very good survival rate but we also know again that this depends on the clinic, on the expertise of the people performing the procedures. In general that would be the oocyte survival. For patients above 38, we know that there is a slight reduction in oocyte survival that could go down to 80% something like that. In general terms if you freeze 10 eggs, you would expect 9 out of 10 surviving the process and be viable for use for IVF.
The survival rates with the vitrification process which is the standard process used now, we always tell patients that about 80 to 90% of the eggs that are thawed will survive. Then, more importantly, those fertilization rates are the same as fresh eggs which are about 90%. So if that egg does survive which is about an 80 to 90% chance it will survive, the fertilization rates are the same as a fresh and the pregnancy rates are the same as a fresh egg.
During the vitrification procedure, we make sure that we have a very high survival rate. And this survival rate of 90% means that 9 out of 10 oocytes are going to survive. And around 7-8 out of 10 oocytes are going to be fertilized. That means that we are very proud of the high survival rate that we can achieve through the vitrification of oocytes.
