Preimplantation Genetic Screening (PGS)

What is preimplantation genetic screening (PGS)?

The examination for genetic analysis using the biopsy method from embryo cells or the body of a mature oocyte is called PGS. This examination, using only cells obtained from embryo biopsy, is applied in IVF treatment. With the embryo screening for aneuploidy in PGS, all 24 chromosomes are scanned and a possible aneuploidy situation is detected.

What is preimplantation genetic testing (PGT)?

The screening test for genetic analysis before the transfer process on embryos obtained by IVF is called Preimplantation Genetic Test. The most important feature of the test is that it makes it possible to determine the genetic source of an abnormal embryo that may prevent the finalization of the pregnancy. In this way, an unsuccessful IVF process is prevented. With the PGT screening test, it becomes possible for the embryo to be selected for transfer to have the correct number of chromosomes.

Another importance of the PGT test is that it helps to prevent embryo transfer, which is unlikely to lead to pregnancy. The risk of carrying a serious hereditary disease before transfer is prevented by PGD.

What genetic tests can be performed?

Examinations on embryos for preimplantation genetic testing can be grouped under three headings;

  • Preimplantation genetic screening for abnormal chromosome number (PGT-A)
  • Preimplantation genetic structural rearrangement test (PGT-SR) performed to detect incorrect chromosomal rearrangements such as translocation and inversion.
  • Preimplantation genetic testing for monogenic (individual) disease (PGT-M)

The main reason for performing tests is to measure the effect of genetic abnormalities in the IVF process, which results in unsuccessful implantation and miscarriage. Another reason is to understand whether there is a genetic disorder due to inherited causes such as muscular dystrophy. If such risky situations are discovered during pregnancy, embryos are not transferred to the mother’s womb.

Who should have PGD or PGS?

PGD ​​is done to detect a specific gene defect. PGS, on the other hand, is done to find out if there is any abnormality, deficiency or excess in the chromosome. In both applications, the main purpose is to detect a situation that may adversely affect the IVF cycle and to contribute to giving birth to a healthy baby.

Preimplantation genetic testing for aneuploidy (PGT-A)

PGT-A, known as the preimplantation genetic test for aneuploidies, makes it possible to screen chromosomal abnormalities. With this test performed on embryos created by IVF method, missing or excess chromosomes are scanned.


How is PGS performed?

Embryos fertilized at the first step of PGS should be cultured within 3 to 5 days. After the culturing process is completed, an embryo biopsy is performed. On day 3 embryo biopsy, one or two cells are removed for testing, and on day 5 embryo biopsy, three or six cells are removed for testing. The biopsy taken from the intra-embryo cells, which are divided into two different types as the cells that will form the fetus and the cells that will form the placenta, are transferred to the laboratory for examination and the results are awaited.

How is PGT performed?

All three variants of PGT are done in a similar way. Embryos are observed in laboratories on the 5th or 6th day, which is defined as blastocyst. A small number of cell biopsies are removed from each embryo and sent to the laboratory for PGD purposes. PGT is performed on the DNA obtained from the cells and the embryos are frozen and stored in the laboratory. If the results are ready (usually 1-2 weeks after the biopsy), information about the results is given. If the embryos are suitable for transfer, the plan is organized with the relevant doctor.

Candidates for PGT-A include

The eligibility criteria for candidates for PGT-A are generally as follows;

  • Being over 35 years old,
  • Having experienced one or more pregnancy losses,
  • Having an unsuccessful IVF process,
  • Having a pregnancy with chromosomal abnormalities,
  • Wanting to balance the genders in the family.

Preimplantation genetic testing for a monogenic disease (PGT-M)

PGT-M, formerly known as PGD, is a genetic diagnostic test performed before the embryos produced in the IVF cycle are transferred to the uterus. With the PGT-M diagnostic method, in which genetic disorders that occur due to a single defective gene are detected, diseases with a risk of inheriting are detected. The PGT-M test, in which all monogenic defects (autosomal dominant, autosomal recessive, etc.) are discovered at an early stage, helps people to have a healthy child.

PGT for chromosome structural rearrangement (PGT-SR)

The PGT-SR test for structural chromosomal rearrangements is performed to identify chromosomal changes that occur due to inversions and balanced translocations. PGT-SR, a genetic test performed on embryos created by IVF, is recommended for translocation or inversion carriers.

Chromosomes, known as the structures that host development and growth, are changed in such a way that their sizes change with the structural rearrangement of chromosomes. Embryo biopsy samples are sent to the laboratory environment within the scope of applications defined as copying, deletion, inversion and translocation. Unbalanced translocations are screened. Only those with the right amount of genetic material are then selected.

How are PGT-A, PGT-SR and PGT-M performed on embryos during IVF?

After the cells are removed from the embryos obtained in the IVF cycle, the genetic material taken from the related cells is examined in the laboratory environment. In all PGD types, embryos must be in the in vitro fertilization cycle in order to be genetically screened. The menstrual cycle starts with the menstrual period after the first appointment determined by the doctor responsible for the process. After 15-20 days of ovarian stimulation, the eggs are collected.

After the collected eggs are grown, they are subjected to a process of removing cells from them by a technique called embryo biopsy. It is then processed to prepare for genetic screening. Embryos are frozen while waiting for results.

Depending on the type of test performed, the report will be ready within 10 or 15 days. In the PGT-SR test performed to detect structural rearrangements, translocations lasting 30 to 60 days may need to be examined beforehand, depending on the chromosomal abnormality.


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