There are two techniques laboratories use to cryopreserve oocytes: the first is slow-freezing, which was first used to cryopreserve human embryos in 1983 and oocytes in 1986. The slow freezing approach worked well for embryos but not for eggs. The second technique to cryopreserve oocytes is vitrification, which was introduced around 2006 and is much more efficient than its predecessor. Today most, but not all, IVF clinics have switched to vitrification.
A major difference between the two techniques is the speed of “cooling”. Slow-freezing cools oocytes at a rate of 0.3 Celsius per minute, whereas vitrification cools at a rate of over 15,000 Celsius per minute. This has a dramatic impact on outcomes when cryopreserved oocytes are used. When we employed slow-freezing for a patient, she often needed to store 20 - 100 oocytes to have a baby. Today, when we use vitrification, that same woman likely needs 4 - 12 oocytes to have a successful pregnancy.
Does vitrification negatively impact oocytes?
Any procedure can “negatively” impact the oocyte (or derived embryo), including vitrification. The most common issue is the oocyte does not survive the cryopreservation process. The typical “survival rate” of oocytes after vitrification is about 85 - 90%, thus for every 10 oocytes that undergo vitrification, eight or nine will survive. When vitrification is done improperly, fertilization rates and embryo quality may also be impacted. However, if oocyte vitrification (and warming) is performed properly (using the right instruments, solutions and carried out by an experienced embryologist), we see no difference in outcomes when we compare the results to oocytes that were not frozen, but instead immediately fertilized and cultured to blastocyst stage.
Below is a table that shows fertilization and embryo development outcomes from patients who divided their freshly retrieved oocytes into two groups (this is called the “sibling egg model”). The first group of eggs were vitrified, then warmed, while the second group of eggs were used “fresh” (no cryopreservation). Fertilization and embryo culture were performed at the same time and using the same conditions. As the results show, there was no difference in fertilization and embryo development rates between the fresh oocytes and the vitrified/warmed oocytes.
Naturally, patients care about pregnancy rates. The below table shows implantation and clinical pregnancy rates in two groups of patients (using donor oocytes). In one group the oocytes were vitrified/warmed, in the other group oocytes were used fresh (no cryopreservation). As you can see, pregnancy rates were similar.
Does vitrification of oocyte lead to potential problems in the child?
We don’t know yet. However, “preliminary data” published in a number of journals have not demonstrated any increased risk to the fetus or offspring (Cobo et al., Obstetric and perinatal outcome of babies born from vitrified oocytes. Fertil Steril. 2014 Oct;102(4):1006-1015). Just like with many other medical procedures, such as IVF or ICSI, we have to collect data for several years to see if there is any increase in risk to children born using these procedures.
Are All Laboratories Good at Vitrification?
There are probably differences in outcomes (oocyte survival, pregnancy rate, etc.) among the different centers. Oocyte cryopreservation (vitrification and warming) is more “operator” dependent than embryo cryopreservation/vitrification. Individual experience and handling is important for optimizing outcomes. I have no hard data on the subject but if I had to guess, I’d presume most IVF centers are proficient when it comes to freezing embryos. On the other hand, when it comes to cryopreserving oocytes, my guess would be that 20% of clinic are excellent, 60% are adequate and 20% have a lot of room to improve. Again, those are just guesses.
How Can I Judge if a Clinic is Good at Cryopreserving Oocytes?
There is not a simple way to answer this question. While most IVF centers have “basic experience” with oocyte cryopreservation, differences cannot be found in an “objective” way. For instance, outcomes using cryopreserved oocytes are not shown in the SART database - except for “frozen donor eggs”.
I think the best approach is to find someone at the clinic, a doctor or embryologist, who can answer the following:
How many patients have had oocytes cryopreserved in the last 12 month at the clinic (or since the start of the “egg freezing” program at the clinic)? In my opinion, you want to hear that the clinic cryopreserves oocytes on a daily basis and not just once in a while.
How many patients returned to use those cryopreserved oocytes, and what was the outcome (oocyte survival, pregnancy rate)? It it is possible that some centers do perform quite a number of oocyte freezes, but if they don’t perform too many egg thaws/warmings, then in fact they may not know how well the oocyte cryopreservation program is working.
Ideally, you want to hear that the clinic performs oocyte thawing/warming on a daily basis, and not only once a week or once every two weeks. Survival rates of oocytes after warming should be at least 80% or higher, and clinical outcomes (pregnancy rates) should be similar to that of “fresh” patients with comparable age and clinical “parameters”. Performing egg thawing/warming only occasionally, and not being able to cite their own outcome data (preferentially published data), may be a sign that the center doesn’t yet have sufficient experience.
Additionally, if the center has an active “frozen donor egg bank” (i.e. if the center is performing donor oocyte vitrification / warming), they likely have extensive experience with oocyte vitrification, including the warming. That is certainly a good sign.
Can vitrified oocytes (or embryos) be transported (from one center to another)?
Yes, vitrified oocytes (and embryos) can be transported from one IVF center to another, across the country (or even between countries), without any problem. In fact, there are a number of donor egg banks that have been shipping vitrified oocytes for many years with no meaningful issues.