With regard to the fact that during the IVF procedure many eggs and usually many embryos are obtained and not all of these eggs undergo fertilization as well as not all embryos can be transferred to the uterus, the remaining egg cells and surplus embryos can be subjected to freezing (cryopreservation) for the purpose of their future use.

  • Freezing the embryos which have not been transferred to the uterus and their later use gives chance of obtaining a pregnancy after embryo thawing.
  • However, the embryos which were frozen not always survive the process of freezing.
  • Today, it is possible to freeze embryos, and the effects obtained after thawing are comparable.
  • In the case of embryo freezing, ethical and legal issues can emerge upon the couple’s separation or divorce.
  • Every couple in possession of frozen embryos should feel responsible for having them.

Embryo freezing (cryopreservation) is a common procedure. Given the fact that stimulation leads to the formation of the higher number of egg cells, and in connection with their fertilization – higher number of embryos, not all embryos can be transferred to a woman’s uterus.

The embryos which have not been transferred to the uterus can be frozen for their future use. This allows to reduce costs and hardship by skipping ovarian stimulation in case the decision is taken on further in vitro fertilization procedures. The possibility to freeze embryos reduces also the patients’ need to transfer many embryos in the same cycle, thus decreasing the risk of multiple pregnancy.

Embryo freezing is also the procedure of choice in the case of hyperstimulation syndrome (in such a case, all eggs or all embryos are frozen) or when the couple knows that in the future their fertility can be limited due to, for example, necessary cancer therapy or surgery.

The rate of pregnancies obtained after embryo thawing and transfer is slightly lower than in case of an IVF cycle with the use of non-frozen embryos. This can be related to the fact that usually the embryos that are frozen have a bit lower quality as the best embryos are transferred immediately in a given cycle.

In cases where the embryos for immediate transfer are not selected and all are frozen, the rate of obtained pregnancies is close to the result obtained in the case of fresh IVF cycles.

  • Indications for embryo freezing:
  • freezing surplus, properly developing embryos in the course of in vitro fertilization procedure;
  • freezing embryos in the course of in vitro fertilization procedure with negative qualification of the patient to embryo transfer in a given IVF cycle;
  • freezing embryos which were not transferred in order to minimize the risk of multiple pregnancy;
  • preservation of reproductive potential in case of necessary medical procedure which could limit it;
  • increasing the chances of achieving pregnancy within one stimulation cycle (first fresh cycle and then transfer of frozen embryos);
  • reducing the stimulation-related risk by decreasing the number of stimulated cycles;
  • reducing the costs of in vitro fertilization procedures (lack of stimulation when frozen embryos are transferred);
  • minimizing the risk of the development of hyperstimulation syndrome in case there is such risk and postponing the beginning of a potential pregnancy.

Embryo cryopreservation techniques and risk associated with embryo freezing

There are several techniques of embryo freezing and they are continuously improved. Currently, these techniques include the so-called slow freezing – gradual freezing of embryos supported by special devices; and rapid freezing, or vitrification.

The most harmful factor in the process of embryo freezing are ice crystals forming inside the embryos. They can damage the fragile inner structures of cells and in the result disturb or even prevent further embryo development.

  • Slow freezing – controlled slow freezing

In order to reduce the formation of crystals, various procedures using special chemical compounds called cryoprotectants are applied. These are usually polyalcohols and monosaccharides: polyethylene glycol, glycerol, glucose, sucrose, and others. Cryoprotectants are intended to protect cells by displacing intracellular water from which crystals form. However, the level of these compounds in a cell cannot be too high as, in such a case, they become toxic for cells.
  • Vitrification

Vitrification is a process of transition from liquid to solid state without the formation of ice crystals unfavorable for cells. This effect is achieved by applying high concentration of cryoprotectants and rapid cooling (at the speed of 1,000-30,000°C/minute). A structure forming during the process of vitrification resembles glass.

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Published: 3 November 2015 Updated: 3 April 2017