Vienna In Vitro Center, Sofia, Bulgaria > We can help > Freezing of cells & frozen transfer



In some treatment cycles where a high amount of embryos are obtained, there are still some left in the culture medium which indeed may lead to a pregnancy. In such cases it is possible to freeze these embryos in order to thaw them at a later stage (not longer than 5 years according to bulgarian laws) for another attempt.

The method used to freeze the embryos is called vitrification, also known as 'fast freezing'. The embryos are first cultivated in special cryoprotective cell media and then moved to liquid nitrogen tanks at minus 196 celsius where they remain for storage. 

The advantage of this method is that due to the speed of the temperature fall there is no time for water crystals to be formed- otherwise these would destroy the embryo. The same method is used also for egg cell freezing, however, egg cells are much more sensitive to the freezing process and therefore even more special consumables have to be used.

Sometimes it might be even necessary to freeze all embryos or oocytes for example if hyperstimulation occurs, not well prepared endometrial layer, bleeding, no sperm cells available etc.

the embryos can be frozen on any stage of their development but the best chance of survival have the blastocysts.

Sperm cells and tissues can also be frozen - in these case different protocols are used.

Sperm cells have very good chances of survivat due to their much smaller size and their high numbers compared to egg cells and embryos.

Is it possible to freeze embryos in each treatment cycle?

In order to ensure further successful attempts with cryopreserved embryos:

  • at least two additional embryos should be available for cryopreservation, both
  • providing a proper embryonic development.

There is no point in freezing just one or two embryos or those that did not develop in a regular and timely manner. We therefore request your general decision to freeze embryos. Should there be a sufficient amount of suitable embryos we will gladly do so. We beg your pardon that we cannot freeze all remaining embryos from each attempt. On the day of the transfer or at the latest two days after the transfer - in some cases we have to wait and see how the embryo develops - you will usually be informed about whether a cryopreservation is possible or not.

How do the embryos develop after defrosting?

Not all frozen embryos develop properly after thawing. Hence, it occurs that we freeze and thaw embryos, however, a transfer is still not possible due to the fact that none of the embryos develop further. The embryos are stored in thin tubes (in pairs or in a threesome). That means two or three embryos are defrosted at the same time. Therefore, if we freeze eight embryos there is a good chance of carrying out one or two transfers, but not eight of them!

Cryo embryotransfer

the cryotransfer is a seperate treatment cycle following another iVF cycle where embryos have been frozen for later use. The uterus has to be prepared with hormonal medications before the transfer and if it reaches the required thickness and pattern a few of the previously frozen and stored embryos can be defrosted and transfered like during the classic transfer.

2-3 ultrasound examinations need to be made before the transfer to make sure the uterus has responded to the hormones meets the necessary requirements.

However not all embryos survive the freezing and the defrosting procedures and in rare cases it could even happen that no embryos are viable and available for transfer. how many embryos can be used after the storage depends on their number and quality before the freezing. For example low quality early embryos have much lower chances than well developed blastocysts. 

The embryos that have survived the cryostorage and can be transfered have approximately the same chance to achieve pregnancy as non-frozen embryos. It is a big advantage to this method that no hormonal stimulation of the ovaries and no pick-up procedure need to be made.

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