Preimplantation Genetic Testing (PGT) is an advanced method that allows for the analysis of cell samples taken from embryos created in the laboratory during IVF, before they are transferred to the uterus. In PGT, embryos are examined for specific genetic diseases, such as single gene disorders like cystic fibrosis or sickle cell anemia.
In contrast, Preimplantation Genetic Screening (PGS) does not look for a specific condition. Instead, it involves a detailed screening of all chromosomes in the embryo to identify any numerical or structural abnormalities.
The primary goal of both methods is to identify and eliminate unhealthy embryos before transfer, increasing the chance of a healthy pregnancy and reducing the risk of miscarriage. These procedures are performed only in high-tech laboratories by experienced embryology teams using specialized equipment.
The Process of Embryo Biopsy
The first preimplantation genetic screening was conducted in 1990, and the technology has rapidly evolved since then. Today, three main methods are used to obtain cell samples from embryos:
- Polar body biopsy examines genetic material from the egg before or just after fertilization and evaluates only the maternal contribution.
- Cleavage-stage biopsy is performed on day 3 of the embryo’s development, typically at the 6–8 cell stage, by removing one or two cells.
- Blastocyst-stage biopsy, now the most widely used, is done on day 5, taking several cells from the outer layer of the embryo (trophectoderm).
Scientific studies show that blastocyst-stage biopsy causes the least damage to the embryo and offers greater diagnostic accuracy. For this reason, our clinic primarily uses the day 5 biopsy approach.
Who Is It For?
Preimplantation genetic testing and screening are recommended for certain patient groups. The risk of chromosomal abnormalities in eggs increases significantly in women aged 38 and older. PGT is especially beneficial for couples who have experienced repeated pregnancy losses or more than three failed IVF attempts.
It is also advised for families with a history of genetic diseases or a previous pregnancy affected by a chromosomal condition like Down syndrome. PGT is appropriate in cases where one parent has a known chromosomal rearrangement (karyotype abnormality), when either partner carries a single gene disorder (autosomal dominant, autosomal recessive, or X-linked), or when HLA typing is required.
In families requiring HLA matching, the goal is often to use cord blood or bone marrow from a healthy baby to treat a sibling with a medical condition.
The most common indication for PGT remains advanced maternal age. A landmark 2014 study by Franasiak et al. demonstrated that the rate of aneuploidy (chromosomal abnormalities) in embryos increases significantly with age. This highlights the importance of genetic screening in improving IVF outcomes.
Genetic Analysis Methods
Cells taken from embryos are analyzed using advanced technologies:
- Array Comparative Genomic Hybridization (aCGH) screens all chromosomes for both numerical and structural abnormalities.
- Single Nucleotide Polymorphism (SNP) Microarrays provide detailed information on genetic variants and can assess familial relationships.
- Quantitative Real-Time PCR (qPCR) is a sensitive method that offers rapid results.
Potential Risks and Mosaicism
Biopsies done on day 3 of embryo development may carry a higher risk of damaging the embryo. Day 5 blastocyst biopsy, on the other hand, allows for the collection of genetic material without harming the embryo.
Regardless of the method used, the average error rate for these tests is about 2%. Most errors are due to a phenomenon called mosaicism, where some cells in the embryo have genetic abnormalities while others remain normal. In many cases, the abnormal cells are naturally eliminated during embryo development or remain only in the placenta, allowing the fetus to develop normally.
Our Approach
In our clinic, the priority during IVF treatment is to support embryos in developing to the blastocyst stage on day 5. At this point, embryos are stronger, the risk of genetic issues is lower, and the chances of successful implantation are higher.
This approach has long been practiced with success by Dr. Arda Lembet and his team.
