Preimplantation Genetic Screening (PGS) is a genetic test performed on embryos during In Vitro Fertilization (IVF). This screening checks for chromosomal abnormalities before an embryo is transferred to the uterus. PGS identifies embryos with the correct number of chromosomes, known as euploid embryos, to improve pregnancy success and reduce risks associated with chromosomal conditions.
Why PGS is Used
PGS is utilized in IVF to identify embryos with an abnormal number of chromosomes, a condition called aneuploidy. Aneuploidy is a significant factor contributing to IVF failure, implantation issues, and early pregnancy loss. Approximately 50% of human preimplantation embryos from IVF cycles are chromosomally abnormal, a rate that increases considerably with maternal age. For women aged 30 and younger, 30% to 40% of screened embryos show irregularities, rising to 70% to 90% in women over 40.
By screening for aneuploidy, PGS helps select chromosomally normal embryos, which are more likely to implant successfully and develop into a healthy pregnancy. Transferring euploid embryos can reduce miscarriage risk, as chromosomal abnormalities cause about 70% of early miscarriages. This also increases live birth rates and can decrease the need for multiple embryo transfers. PGS can detect common chromosomal disorders like Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), and Patau syndrome (Trisomy 13). While PGS focuses on numerical chromosomal abnormalities, it differs from Preimplantation Genetic Diagnosis (PGD), which tests for specific genetic diseases like cystic fibrosis.
How PGS is Performed
The PGS process begins after eggs are retrieved and fertilized in the IVF laboratory, with embryos developing for five or six days to reach the blastocyst stage. At this stage, the embryo consists of approximately 100 to 200 cells. A small number of cells, usually about five, are carefully removed from the trophectoderm, the outer layer that will eventually form the placenta. This biopsy avoids disturbing the inner cell mass, which develops into the fetus.
Following the biopsy, the removed cells are sent to a specialized genetics laboratory for analysis. The embryos are cryopreserved, or frozen, while awaiting results. Modern genetic analysis techniques, such as Next-Generation Sequencing (NGS) or array Comparative Genomic Hybridization (aCGH), examine all 23 pairs of chromosomes for missing or extra copies. NGS is effective at detecting smaller genetic changes and mosaicism. Once results are available, the fertility team identifies chromosomally normal embryos with the highest potential for successful transfer.
Understanding PGS Results
PGS results classify embryos based on their chromosomal makeup, guiding transfer decisions. A “euploid” result indicates an embryo has the correct number of chromosomes (46 total, 23 pairs), considered chromosomally normal. These euploid embryos are preferred for transfer due to their higher likelihood of successful implantation and healthy pregnancy development.
Conversely, an “aneuploid” result signifies an embryo has an abnormal number of chromosomes. Aneuploid embryos are typically not recommended for transfer due to a significantly reduced chance of live birth and a higher risk of miscarriage or severe genetic conditions. Some embryos may show a “mosaic” result, containing a mix of normal and abnormal cells. The implications of mosaic embryos are complex; they may sometimes be considered for transfer, especially if no euploid embryos are available, after careful discussion with the medical team.
Important Considerations for PGS
While PGS offers significant advantages in IVF, several factors warrant consideration. The cost of PGS is an additional expense, varying by clinic and number of embryos tested. This investment may reduce overall IVF costs by decreasing the number of transfers needed. There is also a time delay, as embryos are frozen while awaiting results, typically several days to a few weeks.
Although PGS technology is highly accurate, a small possibility of false positive or false negative results exists. The embryo biopsy carries minimal risk of damage. In some instances, particularly for older women, no euploid embryos may be found, which can be emotionally challenging. PGS is generally recommended for individuals with advanced maternal age (over 35), a history of recurrent miscarriages, or previous failed IVF cycles, but it can be an option for any couple seeking to increase their chances of a healthy pregnancy.