Preimplantation Genetic Diagnosis: PGD (Preimplantation Genetic Diagnosis:PGT) during in vitro fertilization, it is the process of genetically testing the embryos for a specific disorder and then selecting only the healthy ones and transferring them to the mother.

Preimplantation Genetic Screening (PGS): The process is to evaluate the chromosomes in terms of chromosomal anomalies, which we call aneuploidy, before the embryos are placed in the uterus. With the PGS method, it is aimed to detect and transfer embryos without any abnormalities and to have a healthy baby.

Known as aneuploidy, the number of chromosomes being more or less than normal is one of the most common chromosomal anomalies. Preimplantation Genetic Screening (PGS) is the evaluation of chromosomes for aneuploidy before embryos are placed in the uterus. Aneuploidy can occur in both eggs and sperm, as well as in the embryo during the fertilization phase. Aneuploidy may cause physical/mental development problems, depending on the type of chromosomal anomaly present. With the PGS method, it is aimed to detect and transfer embryos without any abnormalities and to have a healthy baby.

While some of the embryos obtained in in vitro fertilization cannot be implanted in the uterus, some may stop developing in the early period, and some may not continue their development until birth even if they survive these processes. As a result, 20% of pregnancies may end in miscarriage, just like natural pregnancies. Chrosomal irregularities account for the largest share among the causes of embryo development failures. The most common chromosomal anomalies seen in miscarriages are: trisomy (presence of three of one chromosome) in chromosomes 13, 16, 18, 21 and 22, monosomy (presence of one copy of the chromosome) and a numerical abnormality in the sex chromosomes (X and Y).

What is PGD?

Preimplantation Genetic Diagnosis (PGD) is applied to couples at risk of cystic fibrosis, thalassemia or balanced structural translocation. Balanced structural translocation results in genetically unbalanced gametes (sperm or sperm), which, if fertilized, can cause deficiencies or excesses in genetic material, resulting in a chromosomally abnormal embryo. Such a situation occurring in the embryo may lead to the death of the embryo, miscarriage, or the birth of a child with serious medical problems.

How and When is PGS/PGD Performed?

After fertilization, the embryo is monitored for 3 days in the culture system until it reaches the 8-10 cell stage. One or two cells from the blastomeres of these 3-day-old embryos are biopsied and chromosomal analysis is performed on specific chromosomes. PGS/PGD procedure is applied to the chromosomes (13, 16, 18, 21, 22, X and Y) where chromosomal anomalies are most common in miscarriages and live births.

While PGS/PGD is performed on chromosomes, special probes (small pieces of DNA) are attached to specific regions to be analyzed by the Fluorescence In Situ Hybridization (FISH) method. Each probe is labeled with a different fluorescent dye. These fluorescent probes are interacted with biopsied cells and expected to hybridize with specific chromosomes. The colored signals (one, two or three) seen on a particular chromosome are evaluated by the geneticist using a fluorescence microscope. A single signal is considered monosomy, two is considered disomy (normal), and three is considered trisomy. Thus, aneuploidy on specific chromosomes can be determined.

What are the advantages of PGS/PGD?

·       Increasing the pregnancy rate by selecting the best embryo to be transferred and deciding on the embryo to be frozen or eliminated
·       Reducing the risk of miscarriage
·        To be able to provide better guidance to patients
·       Encouraging patients who have chromosomally normal embryos but cannot become pregnant

PGS Indications

Advanced age (over 35-37 years old)
recurrent miscarriages
Recurrent in vitro fertilization failure
Having a chromosomal anomaly in a previous pregnancy
Application of Intracytoplasmic sperm injection (ICSI) with sperm with abnormal morphology
Aneuploidy in sperm
Aneuploid mosaicism in at least one of the couples
At least one of the couples is a carrier of X-linked disease

PGD Indications

At least one of the couples is a carrier of rearranged structural chromosomes (translocation, inversion, deletion)
Aiming to achieve a healthy pregnancy by diagnosing a known genetic disease

The PGD method has some disadvantages:

Some technical problems may occur during the preparation or biopsy phase.
Even if a successful IVF and PGD procedure is performed, pregnancy may not occur after the transfer.
The results of analysis of a single cell may be undiagnosed or limited due to mosaicism (due to different contents in the blastomeres of the embryo). Therefore, the result should be confirmed by prenatal diagnosis.
Not all chromosomal and genetic abnormalities can be diagnosed with PGD because only a certain number of chromosomes can be checked.
As a result, only a certain test can be performed on the biopsied cell. All genetic problems cannot be screened with a single cell taken for testing.

What Benefits Does PGD Have?

It increases the success of in vitro fertilization in appropriate cases.
It increases the pregnancy rate in selected cases.
It reduces the risk of pregnancy ending in miscarriage.
It reduces the need for medical termination of pregnancy.
It reduces the rate of multiple pregnancy.
It reduces the economic and psychological burdens of repeated unsuccessful IVF attempts.

Who Should Have PGD?

For older mothers-to-be
Couples who could not achieve pregnancy despite two or more in vitro fertilization treatments.
For couples with recurrent early pregnancy losses (miscarriages) (due to reasons other than translocation carriers)
Balanced translocation carrier pairs
Spouses who are at risk for some single gene diseases that can be diagnosed such as familial Mediterranean anemia, sickle cell anemia, cystic fibrosis, SMA
Selecting HLA compatible embryos with family members
For couples who have a child with a genetic disease from a previous pregnancy
For mothers with a history of pregnancy with aneuploidy (chromosomal disorder)
Gonadal mosaicism (genetic test results of the spouses are normal despite two or more birth products with the same abnormality)
TESE cases (cases with severe male infertility)
Poor responders (cases that respond inadequately to the hyperstimulation protocol)
For diseases transmitted due to the X chromosome, embryonal sex determination if direct genetic diagnosis of the disease in question cannot be made.

Is There a Margin of Error in Embio Biopsy and PGT?
To perform PGT (Preimplantation Genetic Diagnosis), it is necessary to take cell samples from each of the patient’s embryos. The probability of damage to embryos during PGT application is almost negligible and the damage rate is expressed as 0.3%.

Although it varies depending on the method used, the probability of the PGT test giving incorrect results varies between 2-7%.