During the performance of assisted reproduction procedures, the best embryo is selected for transfer into the mother’s uterus. The selection is based on the “look” of the embryo shortly before transfer. As a matter of fact, waiting time normally stretches until day 5 of embryo culture in order to enhance the potential of the selected embryo and its synchronisation with the endometrium.
Sometimes the selection is favoured by performing a Comprehensive Chromosome Screening (PGS/PGT-A/CCS) test. Thanks to this technique, we can find out if an embryo has all the chromosomes in their exact number, in other words, if it is chromosomally normal. In this way, the selection is both morphological (external look) and chromosomal (internal look).
To be able to analyse the embryo we must first biopsy it (take a cell sample) without affecting its development and subsequent implantation. Biopsy techniques have been evolving until the process has been optimised.
Initially, embryos were biopsied on day 3 of culture, when they would reach a number of cells between 6 and 8, so as to minimise the possible adverse effects on its subsequent development. This method involved an excessive manipulation of the embryo.
These days the biopsy technique is performed in the blastocyst stage. This embryo stage is reached on day 5 of culture. During this stage it is safe to biopsy embryos, and the results are more reliable than when it was performed on day 3. Yet, can all the embryos that reach the blastocyst stage be biopsied?
That is probably one of the most difficult questions to answer. A biopsiable embryo is one whose outer layer cells, which will form the placental structures (trophectoderm), can be accessed. In addition, we should be able to collect a minimum number of cells to guarantee a reliable diagnosis. And, as a final requirement, it should be distant enough from the cells that will produce the embryo (inner cell mass) to prevent possible damage.
On this video you can watch an embryo that is qualified for the procedure being biopsied.
Due to the necessary requirements to perform a biopsy, it is possible that on day 5 of culture we do not have any eligible embryos to be biopsied. This does not mean that they are bad quality embryos, but simply that we need to wait until they have reached this special stage. In these cases, embryos will be assessed on day 6 of culture to determine whether it is possible to do the procedure or not. If so, we will biopsy and vitrify the embryos for transfer at the time of highest endometrial receptivity.
Embryo biopsy at the blastocyst stage is a delicate process that requires highly qualified personnel and a good embryo vitrification programme that enhances survival rates after every process.