A good embryo freezing programme is essential to increase the chances of pregnancy in couples undergoing assisted reproduction techniques. This is referred to as cumulative pregnancy rate, or the sum of the chances of pregnancy with one “fresh” cycle then a “frozen” one. In addition, we guarantee excellent results in embryo adoption cycles or when a treatment has to be canceled because of insufficient quality or endometrial bleeding and so the embryos should therefore be cryopreserved.
However, freezing should only be completed when the quality of the embryo is good. Otherwise, we create false expectations among couples as these embryos will not survive the thaw and so will not result in pregnancy. At Instituto Bernabeu, we only freeze A and B embryos in the classification scale recommended by the Spanish Association of Reproductive Biology (ASEBIR). This classification is based on morphologic and kinetic criteria for embryos and is divided into four categories, from the best to the worst (A, B, C and D). Another important aspect is the freezing technique itself. Vitrification currently provides the best results and should be the technique of choice over slow freezing. However a number of considerations that greatly affect the results must be taken into account. The learning curve for embryologists is essential, so extensive training is necessary. Although the protocols are standardised, each laboratory can make small changes that will lead to an increase in success.
When should embryos be vitrified? In reality it can be done perfectly well at any stage. However it is traditionally performed on day 2 and 3 of embryonic development although, as reflected in our statistics, vitrification on day 4 and day 5 offers excellent results.