Embryo cryopreservation

 

Embryo cryopreservation forms an essential part of assisted reproduction treatments as this permits the storage of embryos for subsequent use. Both couples who have failed to achieve a pregnancy and those who have had a pregnancy but may later want to try for a second pregnancy may benefit from embryo freezing.

In IVF cycles, embryos are frozen following embryo transfer in order to save surplus embryos or when the transfer has not been possible for various reasons (medically contraindicated, etc.) and all the embryos retrieved are preserved. This technique is also used in certain cases of preserving fertility.

Embryo cryopreservation is a consolidated technique for embryos in the zygote stage (on D+1 of development) or cell stage (D+2 or D+3 of development) and there is also the option of cryopreservation at the blastocyst stage (D+5, +6 or +7).

ZYGOTES
The cryopreservation of zygotes offers good results in terms of survival (70%-100%), obtaining pregnancy rates around 30%. It is often used in asynchronous oocyte donation where the cycle of the donor is not synchronised with that of the recipient. In these cases, pregnancy rates exceed 40%.

CELLS (D+2/+3)
In IVF cycles, this is the cell stage most commonly used for embryo cryopreservation. Lysis of some or all embryo cells may occur during the freezing and thawing processes. Embryos considered suitable for transfer are those in which at least 50% survive.

Embryo with 4 cells (D+2) pre- and post-thawing Embryo with 4 cells (D+2) pre- and post-thawing

Embryo with 4 cells (D+2) pre- and post-thawing. In the second image, 3 intact cells and one lysed cell are shown. Survival rate 75%. 

3 intact cells and one lysed cell are shown

The success of the technique depends on the quality of the embryo and its capacity to survive the cryopreservation process. The mean survival rate is 80%. The pregnancy rate is around 30-35 %.

BLASTOCYST
Blastocyst cryopreservation is only conducted when the in vitro culture of the embryos is prolonged for 5-7 days. Often in these cases there are not many surplus embryos to freeze as not all embryos reach the blastocyst stage (rate of blastocyst 45-50%).

The choice technique in these cases is often vitrification, reaching survival rates similar to those obtained with early embryos (D+2/+3). The survival rate is around 50%-80% with pregnancy rates exceeding 40% per transfer.

Nearly three decades of experience in freezing human embryos have demonstrated that this technique does not imply any increased risk of miscarriage or malformations.

Legislation

In accordance with article 11 of Law 14/2006 of 26 May on Assisted Human Reproduction Techniques:

3. Following the application of in vitro fertilisation techniques, surplus pre-embryos that have not been transferred to the women in a reproduction cycle may be cryopreserved in an authorised bank for this purpose. Cryopreservation of surplus oocytes, ovarian tissue and pre-embryos may be maintained until the in-charge physicians consider, with a favourable ruling from independent external experts, that the recipient does not meet the opportune clinical requisites in order to be able to conduct an assisted reproduction technique.

4. The different possibilities of use of cryopreserved pre-embryos as well as cryopreserved semen, oocytes and ovarian tissues, as applicable, are:

  • For use by the woman herself or her spouse.
  • Donation for reproduction purposes.
  • Donation for research purposes.
  • Suspension of storage with no further use.


In the case of cryopreserved oocytes and pre-embryos, this final option is only applicable on completion of the maximum storage period established by law if none of the other uses in the previous sections have been selected.

5. The use of cryopreserved pre-embryos or semen, oocytes and ovarian tissues for any of these mentioned purposes shall require the corresponding duly accredited informed consent. In the case of pre-embryos, the consent must be granted by the woman or, if the woman is married to a man, also by the husband, prior to the generation of the pre-embryos.

6. The consent for any of the uses mentioned of cryopreserved pre-embryos or gametes may be modified at any point before being used.
In the case of pre-embryos, the mother or parents shall be requested to renew or modify the previously signed informed consent at least every two years. If it proves impossible to obtain from the mother or parents the relevant consent over two consecutive renewals, and it can be irrefutably demonstrated that the actions taken to try to obtain this renewal were not answered, the pre-embryos then remain at the disposal of the centres where they are cryopreserved to be used according to its criteria for any of the mentioned purposes, maintaining all the established anonymity and confidentiality requirements as well as those of being free of charge and not for profit.

Prior to granting consent, the mother or parents should be informed about the conditions outlined in the previous paragraphs.

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