According to recent information, the percentage of success of in vitro fertilisation cycles may reach up to 70% thanks to embryo vitrification procedures which allow to subsequently defrost and transfer in perfect conditions the frozen embryos throughout the cycle resulting in cumulative gestation rates as high as those mentioned above.

In this way, couples’ chances of success increase significantly, this success rate of 70% would be a cumulative pregnancy rate, a result of combining the fresh cycle pregnancy rate and the frozen cycle pregnancy rate.

Definition of embryo vitrification

Unlike a conventional freezing, embryo vitrification can be defined as a ultra rapid freezing process which has the advantage of avoiding ice crystals inside the embryo. The damage these crystals cause in its cellular structures may harm the viability of the embryo. After a slow freezing the implantation rate is around 33%, while vitrification reaches rates above 50%.

It is generally recognised by embryologists that embryo vitrification produces much better results than slow freezing. Although protocols are normally estandardised, each trading house offers its own protocol, and each laboratory has a way of acting in order to achieve better results.

Embryo vitrification process

In any case, vitrification is more effective both in embryo survival and in implantation rate, which means a higher pregnancy rate with regard to the oldest freezing methods. In addition, a lower number of embryos transferred reduces the likelihood of multiple pregnancy to occur, since rates have improved.

Embryos have a basic state prior to freezing, vitrification must only be performed if there are good quality embryos, given that to act otherwise may create false hopes in couples, since embryos have to bear the freezing process and the defrosting process to reach the frozen embryo transfer with the best chance of success.


Embryo vitrification is very useful when there is a great number of good quality embryos or there are some problem to carry out the transfer. As mentioned before, it is about freezing the excess of embryos of a fresh cycle after transfer so that it allows to increase success rates and to achieve the wanted pregnancy. In case of having good quality embryos after transfer, they could be later vitrified and transferred in the same cycle without stimulating them again.

Embryo vitrification also provides a solution in case of, for example:

  • Transfer cancellation because of inadequate endometrial preparation.
  • A pregnancy may worsens the risk of severe ovarian hyperstimulation
  • Vaginal bleeding
  • Endometrial polyps may hinder implantation
  • Radiotherapy or chemotherapy treatment
  • Any other situation which prevents to perform the transfer

Indications of vitrification

We remind you that oocyte vitrification exists, it may be helpful to patients who must undergo any cancer treatment. Aged patients are more susceptible to this method, women who postpone motherhood or suffer from a low ovarian reserve and wish to accumulate oocytes in order to perform a later transfer. In any case oocyte vitrification is more difficult, since it is about cell survival and to preserve its fertilising potential. Vitrification has achieved to improve oocyte cryopreservation and it will be a workable solution in the future, however it has a long way until its results are comparable with those of embryo vitrification.

Vitrification process

Vitrification process can be performed in any embryo stage, although it is more usual on day 2 or on day 3, but also on day 4 or even in blastocyst stage. The day of vitrification will be chosen depending on the circumstances of the cycle, it is not advisable to generalise.

Vitrification means an extremely fast cell cooling. In case of slow freezing, the cooling rate is approximately -0.3ºC every minute, trying to avoid ice formation inside the embryo. Thanks to a fast cooling vitrification reduces the cell temperature to -15000ºC every minute and avoids ice formation.

Once vitrification procedure has been performed, they will be stored in specific straws in liquid nitrogen banks at 196ºC below zero until they are used.

The time embryos can be kept in storage is limited to the reproductive life of the woman by law. Nevertheless, there is no evidence that the time of storage affect their viability and chances of implantation. If the frozen embryo transfer is not performed, patients are consulted every year about what will be embryos used for. If patients do not want to store them, they can choose to donate the embryos to other couples or to donate them for research.