An Overview of PGT and PGS: Definitions and Conceptual Differences
In the field of assisted reproductive technology, the genetic examination of an embryo before it is transferred to the uterus represents one of the most significant milestones in a scientific journey spanning decades. This process is generally unified under the umbrella term Preimplantation Genetic Testing (PGT). In the past, these procedures were separated into two distinct categories:
Preimplantation Genetic Diagnosis (PGD) and Preimplantation Genetic Screening (PGS). PGD was used to detect known single-gene diseases or structural chromosome abnormalities, while PGS referred to the general screening of embryos obtained from apparently healthy parents for possible irregularities in chromosome number (aneuploidy). However, due to the need to eliminate terminological confusion and the advancement of technology, which showed that the core purpose of both procedures is the detection or screening of a genetic condition, international authorities transitioned to a unified, modern nomenclature.
Now, all these tests are grouped under the PGT title and divided into three subcategories based on the type of genetic material being examined: PGT-A (Aneuploidy), PGT-M (Monogenic/Single Gene Disorders), and PGT-SR (Structural Rearrangements). This modern classification allows both clinicians and patients to understand treatment goals more clearly and precisely specifies the scope of the services offered by genetic laboratories. PGT is fundamentally a laboratory procedure performed by taking a very small cell sample from embryos created through in vitro fertilization (IVF), providing critical information about the embryos’ genetic health. The goal is to select only genetically healthy embryos with the highest potential for transfer, which aims to increase pregnancy rates while reducing the risk of miscarriage.
Historical Development and the Emergence of PGT
The history of preimplantation genetic testing is rooted in the late 20th century, a period of rapid advancement in both genetic science and reproductive technologies. The first PGD attempts were performed in the early 1990s using the simplest genetic analysis methods available at the time, namely Polymerase Chain Reaction (PCR) and Fluorescence In Situ Hybridization (FISH) techniques.
Initially, these tests were conducted by sampling only one or two cells (blastomeres, typically from day-three embryos). These early applications marked a significant breakthrough, especially for couples who were carriers of single-gene disorders, as it provided them with the opportunity to identify at-risk embryos before pregnancy was established, thus avoiding ethical dilemmas such as abortion. However, the first-generation PGT techniques had some important limitations.
The inability of the FISH method to examine more than a limited number of chromosomes (usually 5 to 9 pairs) restricted the scope of aneuploidy screening. Furthermore, the biopsy taken from day-three embryos was later criticized because it could potentially harm the embryo and carried a high risk of misdiagnosis due to mosaicism (the presence of cells with different genetic compositions within the same embryo).
Scientific advancements, particularly the adaptation of array Comparative Genomic Hybridization (aCGH) and subsequently Next-Generation Sequencing (NGS) technologies to PGT in the mid-2000s, revolutionized the test’s sensitivity, speed, and scope. These improvements made it possible to perform the biopsy on day-five or day-six embryos (blastocyst stage), allowing for a larger cell sample and, consequently, more reliable results.
What is PGT-A (Aneuploidy Screening) and Why is it Important?
PGT-A is a screening test aimed at determining whether the embryo’s chromosome number is normal. A normal human cell has 23 pairs (a total of 46) of chromosomes; this state is called euploidy. Aneuploidy is the condition where one or more chromosomes are missing or extra. For example, Down syndrome (Trisomy 21) is one of the most common types of aneuploidy, characterized by an individual carrying three copies of chromosome 21. Aneuploidy in embryos is the most frequent cause of pregnancy loss (miscarriage) and recurrent IVF failures.
The risk of aneuploidy dramatically increases, especially with advancing female age; this risk becomes evident after age 35, and the majority of embryos from women over 40 may be aneuploid. PGT-A aims to significantly increase the chance of a clinically healthy pregnancy by detecting such chromosomal abnormalities before the embryo is placed in the uterus. By utilizing PGT-A results, only euploid embryos (those with the normal chromosome number) are transferred.
This both increases the live birth rate per embryo transfer and reduces the risk of miscarriage or the birth of a child with a genetic syndrome related to aneuploidy. Therefore, PGT-A can offer a safer and more effective path compared to standard IVF treatment for couples with advanced maternal age, a history of recurrent miscarriages, or a history of aneuploidy in previous pregnancies.
How Does PGT-M (Single Gene Disorders) Work and When is it Used?
PGT-M is a diagnostic method used to detect the presence of known hereditary diseases in the embryo that are caused by a mutation in a single gene. This test is applied in cases where one or both parents are carriers of, or affected by, autosomal dominant, autosomal recessive, or X-linked genetic disorders such as cystic fibrosis, sickle cell anemia, spinal muscular atrophy, Huntington’s disease, or thalassemia.
PGT-M, unlike PGT-A, involves the detection of a specific genetic defect, not a general screening. Therefore, the PGT-M process requires comprehensive preliminary preparation (workup) in the genetic laboratory well before the IVF cycle begins. In this preparation phase, DNA samples from family members (parents and, if possible, an affected relative) are collected, and highly sensitive genetic probes or genetic markers (haplotypes) specific to the mutation being tested are developed.
This preparatory work is vital to ensure that the laboratory can make an accurate and reliable diagnosis even when using the very small amount of DNA obtained from the embryo cells. The primary goal of PGT-M is to reduce the risk of passing on such serious hereditary diseases to the next generation to nearly zero. Furthermore, in some cases, PGT-M can also be used for HLA typing (selecting an HLA-compatible embryo for a sibling in need of a stem cell transplant, often referred to as a “savior sibling”). However, this specific application is a practice that generates intense ethical debate.
For Whom is PGT-SR (Structural Rearrangements) Suitable?
PGT-SR is designed for couples where one of the parents is a carrier of structural chromosomal rearrangements, such as translocations (exchanging of chromosome parts) or inversions (reversing of a chromosome segment). These couples are usually clinically healthy because there is no loss or excess of chromosomal material (balanced translocation). However, during the production of germ cells (sperm and eggs), this balanced structure can be disrupted, leading to unbalanced gametes that have missing or extra genetic material. Embryos resulting from the fertilization of these unbalanced gametes usually either fail to implant in the uterus (implantation failure) or cause miscarriage in the early stages of pregnancy.
In rare cases, these embryos can lead to the birth of babies with severe genetic disorders. The critical role of PGT-SR is to examine the embryos obtained via IVF from couples carrying such structural anomalies, identifying only the genetically normal (euploid) or balanced rearrangement carrier embryos. Balanced carrier embryos can be transferred, as these children will be healthy just like their parents but will also be carriers. PGT-SR helps these carrier couples break the cycle of recurrent pregnancy loss and maximize their chances of having a healthy, live baby. This test, like PGT-M, typically requires detailed preliminary workup in the genetic laboratory before the IVF cycle commences.
Steps in the Preimplantation Genetic Diagnosis Process: A Phased Review
The PGT process begins as an integral part of a standard in vitro fertilization (IVF) treatment and requires the high-tech capabilities of a genetic laboratory. The first step is controlled ovarian stimulation, followed by the egg retrieval (OPU) procedure. The collected eggs are fertilized in the laboratory using the male partner’s sperm via the intracytoplasmic sperm injection (ICSI) method. The resulting embryos are carefully monitored by embryologists and cultured up to day 5 or 6; at this stage, the embryos are called blastocysts and are divided into two main cell groups: the inner cell mass (ICM) that will form the fetus and the outer cell layer (trophectoderm) that will form the placenta. The most critical step of PGT, the biopsy, is performed by delicately separating a few cells (typically 5 to 10 cells) from this trophectoderm layer using a laser. Once these cells are separated from the embryo, they are placed in a special tube for genetic analysis. The biopsied embryos are immediately cryopreserved (vitrified) to prevent damage and are stored until the genetic results are available.
Integration of IVF Treatment with PGT/PGS
The integration of PGT/PGS into in vitro fertilization (IVF) treatment is not merely the addition of a technical procedure but a strategic decision that changes the timing and management of the treatment protocol. In cycles where PGT is applied, cryopreservation (freezing) of the embryos after biopsy becomes mandatory. There are two main reasons for this: First, it takes anywhere from a few days to a few weeks to receive the genetic analysis results after the biopsy.
Second, and more importantly, instead of immediately transferring the embryo to the uterus after the biopsy, a separate cycle is awaited to fully prepare the uterine lining (endometrium) with medications. This frozen embryo transfer (FET) protocol allows the transfer to take place in a natural or supported cycle when the uterus is physiologically most receptive. Studies suggest that performing a frozen embryo transfer instead of a fresh transfer can increase pregnancy rates and reduce pregnancy complications, especially in patients undergoing PGT, due to the more optimal uterine environment. This integrated approach combines the genetic selectivity of PGT with the optimization of uterine receptivity to maximize the likelihood of a successful outcome.
Biopsy Techniques and Timing: Which Cells are Examined?
The timing and technique of embryo biopsy are vital for the reliability of PGT. In the early days of PGT, the biopsy was generally performed on day-three embryos (6 to 8-cell stage) by removing a single cell (blastomere). However, it was later understood that removing this single cell could potentially reduce the embryo’s developmental potential and that the probability of an incorrect result due to mosaicism was high.
Today, the gold standard is the removal of 5 to 10 cells from the outer cell mass (trophectoderm), which will form the placenta, at the blastocyst stage (day 5 or 6). Trophectoderm biopsy has a minimal negative impact on embryo viability because it does not harm the inner cell mass (the part that will form the fetus). The cells are typically separated from the embryo using precise micro-manipulator devices with laser assistance. During the biopsy, the surrounding membrane of the embryo (zona pellucida) is breached, and the protruding trophectoderm cells are taken.
This method ensures that the amount of cells taken is sufficient for genetic analysis and guarantees that the embryo to be transferred later is of high quality. Modern genetic laboratories use advanced techniques to highly sensitively amplify and analyze the DNA obtained from these small cell samples.
Innovations in Genetic Analysis Methods: From aCGH to NGS
The genetic analysis methods used in PGT have undergone a major transformation thanks to the rapid development of technology. FISH, the first method used, could only examine specific chromosomes and had a limited scope. The subsequent aCGH (array Comparative Genomic Hybridization) and later-generation cGH methods fundamentally increased the effectiveness of PGT-A by providing the capability to screen all 24 chromosomes (22 autosomes and X/Y sex chromosomes) simultaneously.
However, today’s gold standard in the PGT field is Next-Generation Sequencing (NGS) technology. NGS offers higher resolution, faster analysis time, and the potential for lower cost compared to aCGH. Most importantly, NGS is more sensitive in determining low-level mosaic embryos due to its ability to detect more precise copy number variations (CNV). NGS platforms quantify the numerical changes in chromosomal regions following whole-genome amplification of the limited amount of DNA obtained from the embryo.
This technological advancement has maximized the sensitivity and reliability of PGT applications for PGT-A (aneuploidy), PGT-SR (structural rearrangements), and PGT-M (single-gene disorders), thus providing clinicians with clearer and more reliable results.
What are the Main Indications for PGT/PGS?
While PGT/PGS is not a standard requirement for every IVF treatment, it has a strong indication in specific patient groups to increase the chance of pregnancy and minimize genetic risks. The most common indication for PGT-A is advanced female age (35 and older), as the risk of aneuploidy in embryos increases dramatically in this age group.
Other important indications include couples with a history of three or more recurrent pregnancy losses (miscarriages), couples who have previously miscarried due to aneuploidy or given birth to a baby diagnosed with aneuploidy, and those who have experienced multiple failed IVF attempts (recurrent implantation failure – RIF) despite the transfer of good quality embryos. The indication for PGT-M is clearer: one or both parents are carriers of or affected by a known single-gene disorder (e.g., cystic fibrosis, Huntington’s).
PGT-SR is indicated in cases where one of the parents is found to be a carrier of a structural chromosome rearrangement such as a balanced translocation or inversion. These indications show that PGT is positioned as an effective medical intervention tool in situations where the genetic risk is high or the likelihood of pregnancy failure is increased, rather than a random choice.
PGT in Recurrent Pregnancy Loss and Failed IVF Attempts
Recurrent pregnancy loss (RPL) and recurrent implantation failure (RIF) are among the most challenging and emotionally draining conditions in reproductive medicine for couples. A significant portion of these conditions is caused by chromosomal abnormalities in the embryo, i.e., aneuploidy. In RPL cases, miscarriage occurs after the pregnancy is clinically recognized, and most of these miscarriages are due to the embryo having an incorrect number of chromosomes.
In RIF, despite the transfer of seemingly healthy embryos to the uterus, pregnancy never occurs or ends as a chemical pregnancy. PGT-A offers a powerful tool to determine whether the problem in both situations is an embryo-related genetic defect. Transferring only embryos confirmed to be euploid increases implantation rates and dramatically reduces the risk of the next pregnancy resulting in a miscarriage, especially in RPL patients.
PGT helps these couples reduce the number of attempts, lightening both the emotional and financial burden, and shortens the time to achieving a healthy live birth. Therefore, PGT-A is routinely recommended for patients diagnosed with RPL and RIF, especially if the factor of advanced age is also present.
Potential Benefits of PGT/PGS Application and Ways to Increase Success
The main benefit of PGT/PGS application is that it allows for the genetic assessment of embryo quality beyond morphological observation. This enables doctors and couples to select the healthiest embryo, optimizing treatment efficacy. The potential benefits are multi-faceted: Firstly, it increases the pregnancy and live birth rate per transfer. In patients with a high risk of aneuploidy, euploid embryo transfer with PGT-A can yield better outcomes than standard IVF.
Secondly, it decreases the rate of pregnancy loss (miscarriage), as a large proportion of early miscarriages are caused by chromosomal abnormalities. Thirdly, for couples with a limited number of embryos or those exhausted by IVF attempts, it prevents unnecessary and unsuccessful transfer cycles, reducing the duration and cost of the treatment process.
Finally, PGT-M and PGT-SR prevent the transmission of known serious hereditary diseases to their children, offering couples greater control over their genetic future and eliminating the need for genetic risk-related pregnancy termination. These benefits have made PGT an indispensable tool in assisted reproductive technology, particularly for high-risk patient populations.
Are There Risks and Limitations to This Procedure?
PGT, despite the significant advantages it offers, is a complex medical procedure that involves certain risks and technical limitations. The primary risk is the potential for damage to the embryo during the biopsy procedure. However, it is accepted that trophectoderm biopsy, performed at the blastocyst stage and targeting placenta cells, minimizes this risk when applied by experienced embryologists.
The second important risk is false positive or false negative results arising from genetic analysis. A false positive result means an embryo that is actually euploid is reported as aneuploid and unnecessarily discarded; this causes the couple to lose the chance for a potentially healthy pregnancy. A false negative result, on the other hand, leads to the transfer of an aneuploid embryo as euploid, maintaining the risk of miscarriage or the birth of a baby with a genetic disorder.
While modern NGS techniques have reduced these errors, no test is 100% definitive. Furthermore, PGT only provides information about the genetic conditions tested; it does not offer a comprehensive guarantee against birth defects, intelligence level, or other diseases that may emerge later. Another practical limitation of PGT is that the necessary waiting time for the results mandates performing a frozen embryo transfer instead of a fresh embryo transfer.
The Concept of Mosaicism and Challenges in Interpreting PGT Results
Mosaicism refers to the presence of both genetically normal (euploid) and abnormal (aneuploid) cells within the same embryo. This is one of the biggest challenges in interpreting PGT-A results. The PGT biopsy analyzes only a small sample of the trophectoderm, and this sample may not perfectly represent the entire embryo. That is, despite the detection of aneuploidy in the biopsy sample, the inner cell mass (ICM), which will form the baby, might be euploid, or the reverse may be true.
Thanks to the sensitivity of NGS technology, laboratories can now report high-level mosaic (high proportion of abnormal cells) and low-level mosaic (low proportion of abnormal cells) embryos. The clinical management of these mosaic results is highly controversial. While high-level mosaic embryos are generally not transferred, low-level mosaic embryos may be considered for transfer with limited potential, especially if the couple has no other euploid embryos.
The transfer of these embryos can, in some cases, lead to a healthy live birth because mosaic cells may be eliminated as the embryo develops or may remain confined only to the placental tissue (confined placental mosaicism). However, these transfers must be performed with comprehensive genetic counseling and full disclosure of potential risks, and if pregnancy occurs, confirmation of the results with prenatal diagnostic methods (CVS or amniocentesis) is strongly recommended.
The Ethical and Social Dimension of PGT: Controversial Issues
Like any field intersecting medicine and technology, Preimplantation Genetic Testing brings with it deep ethical and social debates. Central to these debates are the moral status of embryos and the limits of selection. Many argue that embryo selection is an early form of abortion because it leads to the destruction of non-selected embryos. Another major ethical concern is PGT’s potential association with eugenics. While the use of PGT-M to prevent serious diseases is widely accepted, some disability rights advocates suggest that genetic testing could promote discrimination against individuals with disabilities and reduce social acceptance. The limits of selection are also a significant point of contention:
PGT can be used for sex selection (for reasons like family balancing, without a medical necessity) or, in the future, might become available for non-medical traits such as intelligence, height, or eye color (the concept of the “designer baby”). This situation creates ethical dilemmas regarding parenthood and the quest to “perfect” children. Furthermore, the use of PGT-M for HLA typing—that is, creating a tissue-matched sibling to treat a sick child—raises concerns about “instrumentalization” (using a person solely as a means to another person’s end).
Finally, the high cost of PGT is a social justice issue, as this advanced technology may only be accessible to financially well-off couples, deepening health inequalities. All these debates demonstrate that PGT is not just a technical procedure but also a practice that reflects society’s values, morality, and outlook on the future.
The Role of Genetic Counseling and Informed Consent in the PGT Process
In a procedure as genetically complex and ethically sensitive as PGT, genetic counseling and informed consent processes are an absolute necessity. The genetic counselor is obligated to convey the couples’ genetic risks, the goals of PGT, its limitations, potential outcomes (especially uncertain results like mosaicism), and the ethical implications of the procedure in a clear, neutral, and understandable language.
Counseling must also address the couple’s emotional and psychological state, supporting them in making the right decision but never steering them. Informed consent is a legal and ethical requirement that documents the couple’s understanding of all information presented, their acceptance of the procedure’s risks, and the fact that the decision was made with their free will. In PGT-M cases, the sensitivity of the test and the risks of misdiagnosis should be explained in detail, and in PGT-A, the clinical uncertainty of mosaic embryo results should be particularly emphasized.
The genetic counselor’s role continues by helping the couple cope with the test results and providing information about fetal diagnosis options (amniocentesis or CVS) if pregnancy occurs. This process underscores that PGT is not only a biological but also an ethical and personal decision.
What Does the Future Hold for PGT/PGS Technology?
The field of Preimplantation Genetic Testing is constantly evolving, and future innovations promise to further expand the test’s accessibility, reliability, and scope. One of the most exciting future directions is the development of Non-Invasive PGT (niPGT). This technique aims to analyze cell-free DNA (cfDNA) released by the embryo into the culture medium, rather than performing a cell biopsy from the embryo itself. If niPGT becomes as reliable as biopsy-based PGT, the potential risk to the embryo could be completely eliminated, and the test application could be much simpler.
Another development is the integration of PGT with Polygenic Risk Scoring (PRS). PRS aims to assess the risk of embryos for common and complex diseases like diabetes, heart disease, or schizophrenia, which arise from the interaction of multiple genes and environmental factors, not just a single gene. This application is still in its early stages, and the clinical utility and ethical implications of PRS are a subject of intense debate. As advancing genetic sequencing technologies increase analysis speed and decrease costs, PGT may become more widespread.
Artificial intelligence (AI) and machine learning can be used to analyze both embryo images and interpret complex genetic data sets, further optimizing the accuracy and efficiency of PGT results. These innovations hold the potential for future PGT applications to cover not only genetic diseases but also other conditions affecting health and quality of life.