In today’s society, egg freezing has become a trending issue, normally associated with those women who want to delay their maternity. But, what does egg freezing really mean? Let’s try to explain this item clearly from the point of view of the assisted reproduction laboratory.

Oocyte cryopreservation is the term used for what is socially known as “egg freezing”.  Cryopreservation is an option for those who want to preserve their fertility. This process refers to the use of ultra-low temperatures to preserve gametes (oocytes from women and spermatozoa from men). It can also be used to preserve embryos or even reproductive tissues (like ovarian or testicular tissue) down to subzero temperatures in order to stop all biological activity, maintaining the appropriate conditions for future use. This has been proofed of particular help in cases in which the individual had to be treated with aggressive therapies, like those required to fight cancer. In such cases, normally partial extraction of the gonadal tissue (ovary or testis) is followed by cryopreservation until the patient overcomes the clinical situation and is ready to conceive. After extensive research, most assisted reproduction laboratories currently use vitrification, an ultra-rapid freezing technology (-196˚C with liquid nitrogen) that avoids the formation of damaging ice crystals in the cells while ensuring their viability after thawing for future fertilization.

INDICATIONS FOR OOCYTE CRYOPRESERVATION

This technique was initially reserved for those women with specific medical indications who had no other fertility options. Progress made in the field of cancer treatments has contributed to the increase of cancer survival rates in populations. However, these treatments are known to affect negatively the fertility potential. In those cases, oocyte cryopreservation can provide hope for fertility preservation.

Egg freezing is also indicated for cases of premature menopause and, consequently, the loss of ovarian function at an early age. However, it must be considered that some cases of early loss of ovarian function might be due to genetic causes, which may be eventually transmitted to the next generation. That is why every single case must be addressed individually and independently subjected to counselling. This would allow to evaluate whether cryopreservation of oocytes and delayed motherhood is actually a good option.

Nowadays, another application for oocyte vitrification is the creation of donor oocyte banks for egg donation programs. One of the great advantages of this is that menstrual synchronization between donor and recipient is not required. Moreover, leftover oocytes are stored for another cycle.

Furthermore, oocyte cryopreservation is useful in those cases in which the woman has been subjected to ovarian stimulation, and yet the male partner fails to produce a semen sample on the day of oocyte recovery. Additionally, it has also been proposed as an efficient strategy for women who had a low response to the stimulation treatments, so the oocyte number recovered in a cycle is not high enough. Thanks to vitrification, those women can store and accumulate oocytes from multiple ovarian stimulation cycles.

However, as it has been mentioned above, the egg freezing approach is currently catching more attention from society due to the increasing number of women who want to delay their motherhood. This means they would be attempting to preserve fertility ahead of any age-related issue that could decrease it. The reason is that fecundity in women is gradually reduced, but there is a significant decline around age 32, and even more pronounced from 37 onwards. This scientific fact is explained by the natural reduction of the oocyte number in the ovaries (ovarian reserve), which happens naturally and progressively in every healthy woman. So, age at the time of oocyte cryopreservation is crucial for the outcome of the technique.

It is also worth to take into consideration the number of embryos generated during cycles. Since implantation rates have improved considerably and the risks of multiple pregnancy are high, there is a tendency to perform single embryo transfer, when possible. Due to the controversy around the definition of an embryo (meaning legal definition that refers to legal rights of the unborn), cryopreservation of supernumerary embryos has raised ethical and legal concerns in many countries. As a consequence, it is not uncommon to see differences in legislation on these matters between countries. With this regard, cryopreservation of oocytes can bypass such problems, given the fact that an oocyte is by no means an individual or human being when it comes to legal issues.

FACTORS AFFECTING OOCYTE SURVIVAL

The main difficulty the oocyte faces for cryopreservation is its own shape and volume. In particular, the oocyte membrane gets frequently damaged and ruptured, which could lead to the immediate death of the cell. Since the oocyte is a single cell structure, there is no backup for recovery from serious injuries, as opposed to multicellular embryos.

Additionally, women age at the moment of egg freezing is a critical factor, not only because the oocyte number decreases over time, but also because their quality is reduced, as well. In fact, several clinical studies have shown that oocyte survival after cryopreservation is higher in younger women (<35 years old) than in +36-year old women.

RESULTS OBTAINED USING VITRIFIED OOCYTES

Several studies have demonstrated that there has been no increase in the number of anomalies in children born from oocytes fertilized after freezing and thawing. Nevertheless, the live birth rate per patient was statistically higher in younger patients compared to older ones (50% vs. 22.9%), regarding age at egg vitrification. Additionally, when older women go back to their vitrified oocytes, they are also more likely to experience pregnancy complications.

It is important to highlight that society is not yet aware of the importance of age for oocyte vitrification. As a consequence, and even though elective freezing of oocytes for delaying childbearing is becoming one of the most frequent indications for fertility preservation, it is still a decision made at a late stage, after which results may not be as good as expected.

It seems that there is not enough information for women on risks, potential, resources and outcomes. Given the tendency of today’s society to postpone the moment of childbearing, women should be aware of all possibilities and difficulties, the earlier the better. If science and media manage to spread that information so that it reaches young women at the right time, decision-making would be improved and women considering going for oocyte cryopreservation would be able to make up their mind on time before it is too late.

REFERENCES

  1. Zhang L, Yan L, Zhi X, Yan J, Qiao J. Female Fertility: Is it Safe to “Freeze?”. Chinese Medical Journal. 2015;128(3):390.
  2. Allahbadia G, Kuwayama M, Gandhi G. Vitrification in Assisted Reproduction. New Delhi: Springer India; 2015.
  3. Argyle C, Harper J, Davies M. Oocyte cryopreservation: where are we now?. Human Reproduction Update. 2016;22(4):440-449.
  4. Stoop D, Cobo A, Silber S. Fertility preservation for age-related fertility decline. The Lancet. 2014;384(9950):1311-1319.
  5. Cobo A, García-Velasco J, Coello A, Domingo J, Pellicer A, Remohí J. Oocyte vitrification as an efficient option for elective fertility preservation. Fertility and Sterility. 2016;105(3):755-764.e8.
  6. Gosden R, Tan SL, Oktay K. Oocytes for late starters and posterity: are we on to something good or bad? Fertil Steril 2000;74:1057–1058.