The purpose of PGS is to identify whether an embryo carries the correct genetic structure and when an embryo has been cleared as normal, the said embryo can then be transferred to the mother’s uterus. There are two main factors that affect the implantation of the embryo: the embryo and it’s the uterus environment.
An embryo that is genetically healthy is more likely to lead to a viable pregnancy meaning it has a higher chance will attach and go on to a live birth. When In vitro fertilization and genetic testing are combined together, it gives a higher chance to select a healthy embryo. With PGS it can be determined whether there is a numerical or structural problem with the chromosomal structure of an embryo.
Who is PGS suitable for?
All couples wanting a child can theoretically perform genetic screening of the embryos. However, it is not advised to avoid unnecessary costs of the genetic screening. In cases where it is really necessary is when we would be insistent on the genetic testing. In this context, although there is no scientific consensus in some cases, embryologic genetic screening is generally recommended in the following situations:
- Genetic problems known to one or both partners (if they have abnormal chromosome counts: Turner Syndrome 45, X0, Kleinfelter
- Syndrome 47, XXY, translocation carriers etc.);
- Although spouses have normal genetic structure, there is an increased genetic defect seen within the embryos;
- Repeated negative outcomes after a transfer;
- Couples who suffer with recurrent miscarriages;
- Couples who already have children with genetic problems;
- Couples who require IVF because of the females mature age;
- Couples whose sperm have been found to have an increased genetic problem or whose sperm counts are low.
- Can we give information about the genetic structure of the embryo by the PGS method?
- The genetic screening of the embryo can be used to assess the genetic structure of the embryo to determine whether there are the correct number or any structural problems with the chromosomes. So 46 chromosomes are complete and normal, missing or extra chromosomes, are determined as abnormal.
How is a biopsy carried out from an embryo?
A few cells are removed from the embryo and this is carried out via a laser with the guidance of a microscope by the embryologist. The biopsy of the blastomere (usually taken in the embryo 3 days at the 8-10 cell stage) or trophectoderm (outer cell mass of the embryo at the blastocyst stage) is used for genetic screening. A trophectoderm biopsy is preferred because there is less risk to damaging the embryo and an increased chance of determining which embryo is healthy-unhealthy, as at day 3 there is mozaism in embryo and chance to lead to inaccurate results.
What are the tests used for PGS?
Array CGH (Comparative Genomic Hybridization): The genetic material in the cells are obtained by the trophectoderm biopsy which is performed at the blastocyst stage of the embryo development (embryos are 5-6 days old). There are 22 pairs of chromosomes in the body cells and the 2 sex chromosomes (X, Y).
NGS (Next Generation Sequencing): Differences from NGS Array CGH are the technical differences in the examination order; other than this, the biopsy time is the same. By this method, all chromosomes are evaluated. The success of detecting mosaic embryos according to the Array method is higher.
(Mosaic embryo: an embryo with both normal and abnormal chromosomal structures)
Can a diagnosis of a known genetic anomaly (mutation) be established with PGS?
PGS is not sufficient for the diagnosis of diseases due to a specific genetic anomaly (e.g. single-gene disorders such as X-linked diseases, Mediterranean anaemia, muscle diseases etc.). Specific and detailed research is required to determine the gene mutation. And before this research, a preconditioning process called set-up is required to determine exactly which gene to evaluate in the genetic makeup. For the set up process, blood is required from both partners and if they have a biological child then them also, a detailed study of the blood is carried over a period of 1-2 months. For rare mutations, the set-up time may take longer. After the set-up is completed, the IVF treatment can only then be started.
What is the difference between Pre-implantation Genetic Diagnosis (PGD) and
Pre-implantation Genetic Screening (PGS)?
In the PGD method, except for technical differences, a limited number of chromosomes can be screened. For PGD, minimal chromosomes i.e. up to 5-9 kinds of chromosomes after a 1 or 2 cell at day 3 (biopsy staining (FISH) or PCR method to be taken at embryos that are at 8-10 cell stage. In addition, the PGD method is particularly preferred in the diagnosis of certain single gene disorders.