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Análisis cromosómico embriones

PGD, PGS or PGT? The ultimate guide to chromosomal analysis of your embryos

If reproductive medicine is something you are familiar with, you will be aware that some people talk about PGD to refer to the chromosomal study that is carried out on embryos before they are transferred. Other specialists will allude to the terms PGS or PGT and patients might wonder: do all these mean the same? is one better than the other, perhaps more comprehensive?

In this article we explain all of them in detail, with the aim of providing you with all the information you need before starting your assisted reproduction treatment with IVF-Life.

Different nomenclature, same origin

The rapid advances in the field of reproductive medicine, the exponential growth of transnational reproduction -especially in Spain-, or the challenging task of reaching a consensus between scientific associations, are some of the reasons why Preimplantation Genetic Testing has undergone changes in, for example, its name.

Although, as mentioned above, below we will clarify what each of these concepts refers to, it is necessary to highlight that what all these concepts have in common is the nature of their technique, which consists of genetically analysing the embryos before they are transferred to the mother’s uterus.

This goal is achieved by obtaining a biopsy of the embryo, specifically of the trophoectoderm (the part that will develop the placenta) at day 5 or 6 of development.

Subsequently, the sample will be analysed in the genetics laboratory and, after about 2 weeks, the specialists will know which of the biopsied embryos are euploid, i.e. chromosomally healthy and therefore suitable for transfer.

Análisis cromosómico de embriones

PGS and PGD: the precursors to PGT

Historically, although the acronym PGD (Preimplantational Genetic Diagnosis) has been widely used, these genetic studies have been known under different names, where PGS and PGT stand out. These have been established over the years, in part, according to the genetic aspects addressed by each one. In this context, it is worth understanding that chromosomal abnormalities are classified, for example, into numerical and structural.

The former occur when the genetic material has an abnormal number of chromosomes, either more or fewer than the 23 pairs considered “normal”. Structural abnormalities, on the other hand, are those that occur when part of a chromosome undergoes some kind of alteration, such as duplications, deletions, translocations or inversions.

PGS (Preimplantation Genetic Screening for Aneuploidies) provides an analysis of all the chromosomes of each of the embryos resulting from an IVF treatment, in order to rule out any abnormalities present in them. On the other hand, PGD (Preimplantation Genetic Diagnosis) focuses on the detection of specific monogenic diseases (such as cystic fibrosis) or structural chromosomal abnormalities in embryos. It is particularly recommended in cases where one or both parents are carriers of a genetic disease or have a chromosomal abnormality. Its main purpose is to select embryos free of a specific disease.

It was in 2017 when, finally, the most recognised associations specialising in assisted reproduction agreed that PGT (Preimplantation Genetic Testing) would be the new and global way of referring to these processes.

PGT (Preimplantation Genetic Testing) and its broad capacity in the genetic study of embryos

The main characteristic of PGT is that it not only replaces all the previous techniques, but also encompasses all their purposes and organises them according to their specific objective, even though it is still often referred to as Preimplantation Genetic Diagnosis or PGD. For this purpose, Preimplantation Genetic Testing is subdivided into three analyses: aneuploidy, structural anomalies and monogenic diseases:

PGT-A (Preimplantation Genetic Testing for aneuploidy)

The purpose of this preimplantation genetic test for aneuploidies is the same as the former PGS, as it focuses on detecting numerical chromosomal abnormalities in embryos, one of the most common and well-known being trisomy of chromosome 21, i.e. Down’s syndrome. Other common diagnostic examples are trisomy of chromosome 18 (Edwards’ syndrome) and trisomy of chromosome 13, corresponding to Patau syndrome.

Furthermore, as is well known, humans have a pair of sex chromosomes, presented in our karyotype as XY for males and XX for females. This is also linked to syndromes such as Turner syndrome, in which females have a total or partial absence of an X chromosome, or Klinefelter syndrome, characterised by the appearance of an extra X chromosome in males.

Just as have described the similarity of PGT-A to PGS, PGD has been transformed into what we now refer to as PGT-SR and PGT-M.

PGT-SR (Preimplantation Genetic Testing for structural diseases)

As its name suggests, studies the different structural anomalies that may occur in embryos, whether or not they include a loss or gain of genetic material, which may affect the viability of the embryo or the health of the future offspring. As mentioned above, these anomalies may include deletions, duplications, translocations or inversions, an example of which is Prader-Willi syndrome, a rare disease – the result of a deletion – in which part of the genetic material of chromosome 15 is lost.

PGT-M (Preimplantation Genetic Testing for monogenic diseases)

this section of the study analyses the presence of hereditary diseases caused by mutation or alteration in the DNA sequence of a particular gene, in other words, the existence of monogenic diseases.

These, also known as Mendelian inherited diseases, are further classified into 3 subtypes, detailed below:

  • Autosomal recessive disease: This occurs when two copies (one maternal and one paternal) of a mutated gene coincide in the genome of the affected person, even though the parents – even if they are carriers of that copy – do not manifest the disease. The probability of offspring developing the disease if both parents are carriers is 25%. Cystic fibrosis or spinal muscular atrophy are examples of autosomal recessive diseases.
  • Autosomal dominant disease: In this case, only one copy of the abnormal gene is required for a person to develop the disease. There is a 50% chance that offspring will inherit the disease from an affected parent, as is the case with Huntington’s disease and Steinert myotonic dystrophy.
  • X-linked disease: This occurs when the mutated gene is on the X chromosome and can be dominant or recessive. Examples of this type of inherited disease include haemophilia A and fragile X syndrome.

For which patients is Preimplantation Genetic Testing indicated?

On this last point, once we have understood what is currently considered as PGT and how it is carried out, we must mention the cases in which specialists recommend that women or couples incorporate these techniques into their fertility treatment.

Despite the fact that once the specialists have met their patients and studied their case in depth, theory can be different from practice, as, broadly speaking, it could be said that Preimplantational Genetic Testing benefits all couples who undergo assisted reproduction treatment, given the increased risk of generating an aneuploid embryo. Likewise, it is a technique that is indicated for couples with a family history linked to chromosomal abnormalities, for those who have experienced failed IVF cycles or repeated miscarriages, and for those who choose to undergo treatment by transferring a single embryo or simply decide to include it in their process with cryopreserved embryos to increase the probability of pregnancy.

IVF-Life and its role in your path to motherhood

Although by reading us today you have understood the purpose of PGT and the changes that its name and utility have undergone over the years and scientific development, it is worth remembering that in our clinics in Alicante, Donostia and Madrid, our multidisciplinary team specialising in reproductive medicine is awaiting you. In addition, during this month of May you will be able to benefit from our financial support project for treatments that include PGT.

Preimplantation Genetic Testing has been essential in bringing to the world many of over 10,000 babies that have been born thanks to our know-how and, therefore, we would love to help you and tell you everything that this technique can do for you and your future family.