PGT-A or PGT?
In pre-implantation genetic testing for aneuploidy screening (PGT-A) a number of or all pairs of chromosomes in one or more cells of the embryo are counted. Embryos with an abnormal number of chromosomes are called aneuploid, hence this technique is also called ‘aneuploidy screening’. Aneuploid embryos are not replaced because they are not viable or can lead to giving birth to a child with problems.
- What is the difference between PGT and PGT-A?
- When does the analysis of the embryos and embryo transfer take place?
- Which chromosomes can be tested?
- What is the benefit of PGT-A?
In PGT the geneticist is looking for a well-defined hereditary genetic defect: the lab knows beforehand which defect on which chromosome or gene is being looked for.
In PGT-A, on the other hand, it is not a well-defined gene or chromosome that is being evaluated, but the chromosomes are counted in the cells removed from the embryo. Thanks to PGT-A a selection can be made in the lab of fertilised embryos, not only on the basis of their morphology (the shape and number of cells), but also of their chromosomal content.
In PGT-A it is preferred that the genetic analysis is carried out on day five after fertilisation because then several cells and therefore more DNA can be studied. It is also possible that the PGT clinic will carry out a biopsy on day three and then freeze the embryos on day five.
In all cases the placement of the chromosomally normal embryos is planned for a later, non-stimulated cycle. Nevertheless the geneticist will first discuss the result of the screening with you.
However, if there are only a limited number of embryos available on day three of the development, there is the possibility – in consultation with your doctor – of not carrying out genetic screening and of placing the available embryos back in the womb.
At the moment we use almost exclusively a very recent technique, next generation sequencing (NGS), which allows us to count all chromosomes. The technique makes it possible to measure the quantity of DNA in the embryo biopsy and to deduce from this whether the number of chromosomes present is normal.
Exceptionally we use the conventional technique, fluorescence in situ hybridisation or FISH , this works with stained DNA markers and allows the analysis of chromosomes 13, 16, 18, 21, 22, X and Y. These chromosomes are often at the root of miscarriages or are involved in a number of serious abnormalities such as trisomy 21 (Down’s syndrome).
Today there is a great deal of discussion about the benefit of PGT-A. A number of studies have shown that the likelihood of pregnancy is not increased by it. In a number of cases PGT-A can however help to better understand the failure of IVF. The new techniques of genetic screening with the help of massive parallel sequencing are nevertheless very promising. They are more accurate and allow all chromosomes to be analysed.
In any case further studies are needed to confirm the added value of aneuploidy screening in the context of an IVF treatment.
If your doctor prescribes PGT-A because this may be beneficial for you, the PGT clinic will try to screen as many embryos as possible genetically. Depending on the number, the quality and the genetic condition of your embryos, it is, however, always possible that no embryos can be replaced.