As we know, embryonic development lasts between 5-6 days. During these days the embryo evolves from the fertilised oocyte, 1 cell, to approximately 150-200 cells. On day 5-6 of development it reaches the blastocyst stage where we can observe two well differentiated structures, the inner cell mass, the cell group within the embryonic cavity that will give rise to the foetus and the trophectoderm, which is the cell layer that surrounds the inner cell mass and which will join the endometrium to form the placenta.
Approximately 40-60% of developing embryos on day 3, where the number of cells is between 6 and 12, reach the blastocyst stage. Although it is true that most of the embryos generated develop until day 3 of culture, where the oocyte plays a fundamental role, from day 3 onwards the embryonic genome is activated and it is the embryo itself that directs its development.
Many of the embryos do not continue their development from day 3 to day 5. Therefore, development up to day 5 of development is in itself a process of embryo selection. Many times we have precious embryos at day 3 that we would transfer and when we leave them in culture we see that they are arrested in their development and would never have achieved pregnancy if we had transferred them.
Embryos that reach the blastocyst stage have a higher probability of implantation. It is also important to bear in mind that nowadays both the culture media and incubation systems and a good blastocyst culture programme in the laboratory allow us to carry out this type of in vitro selection.
