Preimplantation genetic diagnosis (PGD) is the general name given to the tests performed on embryos before they are transferred to the uterus.
This test aims to detect and prevent the transmission of serious genetic diseases or chromosomal abnormalities to the future baby. This technique makes it possible to select embryos that are not affected by the genetic conditions being tested for.
This technique consists of performing an embryo biopsy in which some cells are removed from the developing embryo (around day 5 of its growth).
Genetic material is taken from inside these cells to be analysed to identify whether the disease in question is present. This allows us to select an embryo free of the disease and transfer it to the future mother.
DIAGNOSIS
Preimplantation genetic diagnosis or PGD has been given different names depending on the type of abnormality being investigated. Since 2017, there has been a new consensus on terminology among the world's leading assisted reproductive technology associations.
The term PGT (preimplantation genetic testing) is therefore used, followed by a letter indicating the type of abnormality being investigated.
PGT-A is used to detect numerical chromosomal abnormalities (aneuploidies).
The normal number of chromosomes is 23 pairs, 22 plus the sex pair: XX or XY. An example of a numerical chromosomal abnormality is Down's syndrome, in which there are three copies of chromosome 21 (trisomy 21).
Other aneuploidies that may be found in live-born babies include trisomy 18 (Edwards syndrome), trisomy 13 (Patau syndrome), Turner syndrome (in females, the total or partial absence of one X chromosome) and Klinefelter syndrome (in males, the presence of an extra X chromosome).
There are several types of structural chromosomal abnormalities, which may occur with or without a gain or loss of genetic material, including deletions, inversions, translocations, duplications, insertions and rings chromosomes.
Some of these chromosomal abnormalities may compromise embryo viability, leading to implantation failure or miscarriage, while others may result in a genetic condition in the baby.
PGT-M is used to detect inherited diseases caused by a mutation or alteration in the DNA sequence of a specific gene.
Also known as Mendelian inherited diseases, they can be of three types: autosomal recessive diseases, autosomal dominant diseases and X-linked diseases.
In addition to offering all the advantages of traditional PGT-A, this new system makes it possible to determine the origin of the aneuploidy, identifying whether it comes from the egg or the sperm. This information can help determine whether egg or sperm donation may be appropriate.
In addition, it allows us to confirm whether the embryo has indeed been made with the egg and sperm of the couple and to recover true embryos with two pronuclei, a type of embryo that can be valid for transfer and that are usually discarded. This may increase the number of embryos available for treatment.
Whether preimplantation genetic testing is appropriate in each individual case should always be determined through specialised medical and genetic counselling.
PATIENTS
Dr. Elisa Pérez, from IVF-Life Donostia
FIND ANSWERS TO YOUR QUESTIONS
Chromosomal abnormalities can be divided into two types: numerical and structural.
These diseases, also known as Mendelian inherited diseases, can be of three types:
PGT can help select embryos without the genetic abnormalities being tested for, but it does not increase IVF success rates in all cases or guarantee pregnancy.
PGT is not necessary in all IVF cases, and its use should be determined based on each patient’s individual circumstances and medical history.
WHAT OUR PATIENTS SAY