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The success of transfers with frozen embryos

Embrióloga Madrid

One of the questions that patients ask us in consultation is whether transfers with frozen embryos are successful. At Ovoclinic we have prepared this article to clear up all doubts about freezing embryos, since we understand that there is fear in considering that there is a worse “effectiveness” by having to go through a devitrification process.

All this comes from the fact that, for years, the technique that we had in IVF (In Vitro Fertilization) laboratories to freeze embryos called “slow freezing” was less effective and, in some stages of development (specifically in eggs and blastocyst) the survival rate was lower than desirable.

The arrival of the vitrification technique marked a before and after in embryo cryopreservation.


To answer this question we have to understand that the great drawback of any type of freezing is the formation of ice. We start from the basis that cells contain a very high percentage of water inside. As the temperature drops, this water will form ice crystals that break membranes of internal structures, giving rise to embryonic degeneration due to non-survival.

The strategy of the different cryopreservation techniques goes through the replacement of this intracellular water by substances called cryoprotectants that do not form ice crystals when the temperature drops, thus protecting the cellular structures and allowing the recovery of the embryonic integrity exactly as it was before cryopreservation.


  • Embryos “surplus” from an IVF or ICSI cycle

After the extraction of the eggs in the ovarian puncture, fertilization takes place and this results in the generation of embryos that will be kept in culture until day five – blastocyst – in the incubators of the laboratory. During these days of culture, embryo selection is carried out (only those embryos that have the best chance of reaching a positive result will develop in the appropriate way, advancing through the different stages of development: cells, morula and blastocyst).

Usually around the fifth day, several of these embryos have completed their development favorably and are blastocysts of good quality. One of them is selected for transfer to the uterus and the rest have to be cryopreserved (we cannot keep the embryos in the incubator beyond this embryonic stage).

We freeze all the embryos that have generated and developed favorably giving rise to blastocysts when for medical reasons we cannot perform the embryo transfer. Sometimes, the hormone levels that result after ovarian stimulation are not the most appropriate for the achievement of pregnancy, decreasing the chances of pregnancy or may be harmful to the health of the patient and for the evolution of the future pregnancy. This occurs, for example, if the ovarian response has been high (risk of ovarian hyperstimulation) or if there are hormonal/clinical changes that indicate that the endometrial receptivity is not adequate and, therefore, the conditions are not favorable for achieving pregnancy.

  • Embryos in cases of genetic diagnosis or screening

It is known that in approximately 90% of cases of non-pregnancy or miscarriage, the cause is embryonic due to alterations in the chromosomal endowment of the embryos that prevent correct development. These alterations are due – in most cases – to advanced maternal age. The social and labor circumstances of developed countries have led to a delay in the age at which women seek their first pregnancy and this has a negative impact on its chances.

For this reason, PGD (Preimplantation Genetic Diagnosis) is a technique that, today, is part of our routine in the IVF laboratory and through which we can carry out genetic studies on embryos.

In order to know the genetics of an embryo, we must take a small sample of cells from it, a biopsy, which will be analysed by geneticists. After the biopsy, those embryos that are going to be analysed are frozen while waiting for the result to, in this way, transfer the appropriate embryo to the uterus.

Embryo survival

If the vitrification technique is carried out correctly and the embryos that are selected to be vitrified are of good quality, the chances of these embryos surviving thawing are practically 100% and their behavior is the same in terms of pregnancy rate as the embryo. they would have had if they had been transferred fresh.

In other words, vitrification is a safe technique that allows us to preserve embryos and eggs to optimize the long-term treatments of our patients.

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