01From fertilization and until the embryo transfer takes place in the womb, embryos follow a development that is valued by embryologists daily. Those embryos that have kept a correct evolution and are in better condition are selected to be transferred.

This assessment is performed essentially following morphological criteria (cell number, degree of fragmentation, presence of vacuoles … etc.), and taking into account the day of embryo development in which they find themselves. However, there is great heterogeneity due to the existence of different endpoints and subjectivity derived from the observation of embryologists. The Association for the Study of Reproductive Biology (ASEBIR) established in 2007 a ​​classification with four categories into which the embryos are classified before the transfer. This classification is based on the results of multicentre studies in national reproduction centres and published scientific literature. Based on them, the four categories established are:

A Category: Optimum quality embryo with maximum implantation ability.

B Category: Good quality embryo with high implantation ability.

C Category: Ordinary quality embryo with low implantation ability.

D Category: Poor quality embryo with very little chance of implantation.

02 It is important to point out that, although the embryos are valued daily, the classification of each one of them into the four ASEBIR categories is done the day of the transfer and not before. A part of the morphological, kinetic or embryo development is taken into account in this classification may be that a good quality embryo in a given day of development becomes classified as an embryo of low quality at the time of the transfer. It may also take place the opposite process, finding embryos enhancing of category.

Another important aspect to consider is that none of the embryo classification ensures the likelihood of pregnancy. Neither type A embryo guarantee success, nor a type D embryo ensures failure. And there are many factors influencing the process of embryo implantation.

Mariló Pérez, biologist at Instituto Bernabeu

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Criteria for embryo classification
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