Preimplantation genetic diagnosis (PGT), is the analysis of the embryonic genetic composition before their transfer into the mother’s uterus.
This analysis can identify alterations in the chromosomes of the embryo that could lead to syndromes or diseases in the newborn or even be incompatible with life, causing both early and late miscarriages.
The genetic pool of humans consists of 23 pairs of chromosomes. 22 pairs are called autosomal and one pair determines the sex (XX for women and XY for men). The alteration of these chromosomes, both in their shape (the loss or gain of material, the inversion of a part of the chromosome or the exchange of genetic material between chromosomes) and in their number (loss or gain of one or more chromosomes) can cause serious syndromes in the newborn.
Today, thanks to technological advances, we have techniques that allow us to diagnose many of these alterations.
In order to to carry out a PGT, it is necessary to perform an in vitro fertilization cycle, in order to have the embryos in the laboratory to proceed to their biopsy on the fifth day of embryonic development. Once embryos have been biopsied, they are vitrified, in order to wait for the result of the analysis.
When we have the result of the analysis, we can choose the healthy embryos and we can prepare the patient for the embryo transfer, either in her natural cycle or in a substituted cycle.
There are different options of PGT. PGT-A is used when an analysis of the 23 pairs of chromosomes is carried out to determine aneuploidies (alterations in the number of chromosomes, as in the case of Trisomy 21 that causes Down Syndrome). The PGT-M is performed to detect the presence of monogenic diseases, that is, those that appear with the mutation of a single gene (as in the case of the CFTR gene, the mutation of which is responsible for cystic fibrosis). The PGT-SR is used when we want to detect structural anomalies in chromosomes (such as translocations).
Each PGT has its precise indications and it is essential to correctly advise patients in order to guide treatment appropriately.