The aim of PGT-A is to select an embryo with the correct number of chromosomes for embryo transfer, thereby significantly increasing the chance of pregnancy and reducing the risk of pregnancy loss. Modern NGS sequencing technologies, combined with a multi-cell biopsy of the trophectoderm, allow us to assess the chromosomal constitution of the examined embryos with high resolution. However, these modern approaches have certain pitfalls that we have to deal with. One of the challenges is chromosomal mosaicism, a situation in which the embryo comprises healthy cells and cells with the wrong number of chromosomes. Thanks to better resolution, we can see that up to 15 % of embryos can be affected by chromosome mosaicism. Another relatively common phenomenon is segmental aneuploidy, where part of the embryo's chromosome remains or is missing.
The evaluation of the chromosomal status of the embryo is also very much influenced by the quality of the sample itself, which contributes even more to the large variability of the evaluated data. To maintain consistency in the evaluation, we have introduced a new evaluation system for categorizing embryos, considering our many years of experience and the results of internal studies (see image above). One of these studies was published last year in a prestigious foreign journal. Embryos are now divided into three groups on the basis of PGT-A results. The first group is embryos suitable for transfer (green), which includes euploid embryos and embryos in which it was impossible to evaluate low percentage mosaicism due to the poor quality of the sample. Embryos from this group are always transferred first. The second group is embryos unsuitable for transfer (red), i.e., aneuploid embryos.
The third group is embryos that can be transferred after consultation with a clinical geneticist. This group includes mosaic embryos, which, according to our internal study and studies from other centers, can lead to the birth of healthy offspring. It also includes embryos with segmental aneuploidy. These can be re-biopsied and re-examined, and if the re-biopsy does not confirm this finding, the embryo can be transferred. Read more about this in our article. There are always risks associated with transferring orange-category embryos. For this reason, it is always advisable to consult a clinical geneticist who will assess all the possible risks and explain everything in detail to the patient.
For some patients, an embryo transfer from the "transferable after genetic counseling" group may be the last hope of conceiving a healthy child. These are mainly cases where another IVF cycle is impossible for health, time, or other reasons. According to our statistics, including embryos from this group increases the number of embryos suitable for transfer by 12-13% for women over the age of 35.