Frozen embryo transfer

The improved techniques in embryo freezing and thawing have allowed embryo cryopreservation and subsequent delayed embryo transfer with regard to ovarian stimulation and in vitro fertilization to become an integral part of the ART.

Embryo cryopreservation not only allows us to preserve “supernumerary” embryos to maximize the rates of ovarian stimulation cycle after the transfer of fresh embryos, but it is also an indispensable technique in certain medical or personal situations:

  1. It delays the transfer in specific medical conditions: risk of ovarian hyperstimulation syndrome, or existence of intracavitary pathology, among others.
  2. It allows you to delay pregnancy in particular situations: Personal demands to delay motherhood (personal, medical, economic reasons …).
  3. It enables you to decrease the number of embryos transferred in each transfer: To minimize the risk of multiple pregnancies and maximize the effectiveness of the treatment cycle.

With the high current rates of embryo survival and pregnancy the difference between a fresh and frozen transfer is no longer so evident and pregnancy rates should be considered on an overall basis, including rates with cryopreserved embryos, and then talk about cumulative pregnancy rates.

Technical aspects of embryo cryopreservation

The concept of embryo cryopreservation is to cool the embryos to very low temperatures (-196 ° C in the case of liquid nitrogen) as a way to stop the cellular processes in order to preserve them for an indefinite time and subsequently be reheated so that they can resume their cellular functions and may then be transferred.

To this purpose, the so-called cryoprotectants have been used, substances which for their properties protect the cells from the effects of temperature changes. The specific characteristics of each embryo or cell (volume ratio, volume / surface, water content and membrane permeability) determine the rate of freezing and the pattern of cryoprotectant.

There are two techniques to cool embryos: slow freezing and vitrification, which differ primarily in the application and concentration of cryoprotectant, and the rate of temperature decrease.

    1. Slow freezing:

Slow freezing tries to minimize the formation intracellular ice crystals in a balancing process between cellular dehydration with different concentrations of cryoprotectants, while the temperature decreases progressively (0.3-3 ° C / min) up to – 30 ° C, until finally being immersed in liquid nitrogen (-196 ° C).

    1. Vitrification:

Very rapid freezing using high concentrations of cryoprotectant, which solidify without forming crystals, with direct immersion in liquid nitrogen. There is an extreme rise in the viscosity of the solution (glassy state), hence the name “vitrification”.
In either case the embryos are kept in nitrogen (liquid or vapor). The storage system can be open (direct contact of the nitrogen with the embryos) or closed (sealed prior to their immersion in nitrogen).

Numerous studies have proven the results comparing slow freezing and vitrification with regard to survival rates and embryonic damage (blastomere lysis, embryonic lock). It has been shown that vitrification is associated with a higher survival rate and a higher percentage of intact embryos after devitrification in contrast to slow freezing and thawing in any embryonic state, which makes it possible to have more developed embryos and a lower rate of cryotransfer cancellation. Due to this fact, and because of the technical aspects such as the fact that vitrification is a faster and simpler process, vitrification has become the technique of choice in the embryonic cryopreservation.

At our clinic, thanks to the routine introduction of vitrification, we have gone from a rate of 30% for cryotransfer cancellations due to absence of evolved embryos, to 8%.

How the endometrial preparation is carried out

The preparation phase of the patient’s endometrium is essential for the implantation of the embryos from cryopreservation. To have a suitable endometrial receptivity for the embryos, there must be synchronization between the days of life of the embryo or embryos and the days of the secretory phase of the endometrium. To achieve this synchronous endometrial development, there are several options:

  1. Natural cycle or stimulated cycle: In this case the patient must undergo numerous ultrasound and laboratory controls to monitor the follicular growth (naturally or with gonadotropins, in stimulated cycle) in order to determine when ovulation occurs (with or without hCG), and thus increase progesterone naturally.
  2. Substituted Cycle: Treatment is started with fixed high doses or ascending doses of oral or transdermal estrogen from the first day of the cycle (in patients with ovarian function) or at anytime (menopausal patients). The duration of this artificial follicular phase can vary, and you should add progesterone (vaginally, orally, intramuscularly) on the most suitable day for synchronization. This method presents the disadvantage of possible ovulatory leaks in patients with ovarian function, which will produce early secretory changes in the endometrium, making it impossible to carry out an implantation due to endometrium-embryo asynchrony.
  3. Cycle substituted with pituitary blockage: This involves the administration of a GnRH agonist (normally in depot form) in the luteal phase of the previous cycle, in order to subsequently start, with the onset of the menstruation cycle, a substituted cycle as mentioned in the previous paragraph. Thus potential ovulatory leaks are avoided, allowing the follicular phase to be prolonged by up to 7 weeks or more, without impacting negatively on pregnancy rates.

Cycle substituted with pituitary lock and oral contraceptives: Consists of programming the agonist injection in a controlled contraceptive cycle. This mode is particularly suitable for women with very irregular cycles.

There are several studies and meta-analyses comparing the different patterns of preparation. In patients with regular cycles, there seem to be no differences in the clinical pregnancy rate between natural cycle, stimulated cycle and substituted cycle. In the substituted cycle with pituitary blockage, the necessary checks before the transfer are reduced, which is advantageous over others.

At EUGIN, we use the substituted cycle in menopausal patients, and the substituted with pituitary blockage and oral contraceptives in patients with ovarian function, in order to schedule the treatments better according to the needs of the patient.


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Last Updated: November 2017