Among the techniques for the Preservation of Fertility, the vitrification of oocytes is extremely useful, as it allows the reproductive capacity of a woman to be postponed during the time desired, with the same chances of success as at the point when the oocytes are vitrified.
Vitrification is a process of ultra-fast freezing, in which a liquid turns into a very viscous solid with a glass-like consistency. The advantage of this technique is that the high speed of cooling prevents the formation of ice crystals which can damage the ovum by harming the cellular structures, in contrast to slow-freezing procedures.
Avoiding this possible damage is important because although tissues can compensate for cellular loss up to about 50%, with an oocyte, because it is a single cell, it is a case of “all or nothing”.
The steps are the same as in an IVF cycle: ovarian stimulation with hormones followed by aspiration of the oocytes, but instead of inseminating and fertilising these, vitrification of the oocytes (using the Cryotop method) is carried out, and these are then stored in liquid nitrogen.
This is recommended in numerous and very wide-ranging situations, but the common denominator is postponing the insemination of the oocytes and/or pregnancy.
The oocytes can be kept cryopreserved for as long as the patient wants or needs them, there is no time limit to this.
There are many different devices, but the Cryotop method is the best technique currently available and represents the best option for creating oocyte banks.

When it is recommended

The vitrification of oocytes would be recommended for:

• Women who for whatever reason want to postpone motherhood, in order to avoid the effects of age.
• Oncology patients and others who are going to be given gonadotoxic treatments.
• Women who have had repeated ovarian surgery, for example in the case of endometriosis, as the loss of ovarian tissue and the use of electrocoagulation during surgery leads to a depletion of the follicular reserve, making premature ovarian failure more likely. 
• Patients for whom we would prefer to carry out embryo transfer in a cycle other than the one in which follicle stimulation takes place (when there is a risk of OHSS, development of polyps, hydrosalpinx or hydrometra, absence of spermatozoa, etc.).
• Accumulating oocytes for patients who have a poor response, or in order to obtain a sufficient quantity of these if the objective is to carry out a cycle of pre-implantation genetic diagnosis.
• Avoiding the ethical and legal dilemmas surrounding the freezing of embryos.
• Creating egg banks.

Results

With the vitrification of oocytes using the Cryotop technique, survival figures up to 97% have been achieved for young patients (<35 years old), with pregnancy rates of 65% and implantation rates of 40%.
No statistically significant differences were observed, in comparison to a group of fresh oocytes, either in the rate of fertilisation or in embryo quality. The data concerning the pregnancy and implantation rates are comparable with the results achieved in our oocyte donation programme using fresh oocytes.
On analysis of all ages, it has been observed that the increased age of the mother negatively affects survival, although the clinical results obtained from the vitrification of oocytes are similar to what would be expected if fresh oocytes were used with the same age groups.
Neither have any differences been observed in the weight of live new-born babies in comparison to the results obtained with fresh oocytes, for either single or twin births.