Preimplantation Genetic Screening (PGS), also referred to as Preimplantation Genetic Testing for Aneuploidy (PGT-A), is a genetic test performed on embryos created during in vitro fertilization (IVF). The aim of PGS is to identify chromosomal abnormalities in these embryos before they are transferred to the uterus. This screening process helps in selecting embryos with a normal number of chromosomes, potentially improving the chances of a successful pregnancy. PGS is a screening tool, not a diagnostic test, meaning it identifies risks rather than providing a definitive diagnosis of a condition.
Why Embryos are Screened
The primary purpose of PGS testing is to identify embryos with an abnormal number of chromosomes, a condition known as aneuploidy. Aneuploidy occurs when an embryo has too many or too few chromosomes, deviating from the typical 46 chromosomes arranged in 23 pairs. This chromosomal imbalance is a significant concern because it can lead to various issues, including failed implantation, miscarriage, or the birth of a child with a genetic condition. Aneuploidy is a leading cause of early miscarriages and unsuccessful IVF cycles.
PGS screens for common chromosomal conditions such as Down syndrome (Trisomy 21), Edwards syndrome (Trisomy 18), or Patau syndrome (Trisomy 13), which are characterized by an extra copy of a specific chromosome. Many other chromosomal abnormalities can prevent an embryo from developing successfully. The goal of screening is to identify embryos with the correct number of chromosomes, known as euploid embryos, to enhance successful implantation and a healthy live birth, thereby optimizing IVF success rates.
The Testing Process
The process of performing PGS testing involves several precise steps, beginning with the embryo biopsy. A few cells are carefully removed from the trophectoderm, which is the outer layer of a blastocyst-stage embryo that will later form the placenta. This procedure is performed on day five, six, or seven of embryo development, when the embryo has matured into a blastocyst. The removal of these cells is considered safe as it does not affect the inner cell mass, which develops into the fetus.
Once the cells are biopsied, they are sent to a specialized laboratory for genetic analysis. Common techniques used for this analysis include Next-Generation Sequencing (NGS) and array Comparative Genomic Hybridization (aCGH). These technologies count the number of chromosomes present in the biopsied cells to determine if the embryo has a normal chromosomal complement. After biopsy, embryos are vitrified, or flash-frozen, and stored while awaiting genetic analysis results. This freezing allows time for genetic assessment before embryo transfer.
Interpreting the Findings
The results of PGS testing categorize embryos primarily into euploid, aneuploid, or mosaic. A euploid embryo has a normal number of chromosomes, meaning 46 chromosomes in 23 pairs. These embryos are prioritized for transfer due to their higher potential for successful implantation and development into a healthy pregnancy. Conversely, an aneuploid embryo has an abnormal number of chromosomes, either missing or having extra chromosomes. These embryos are not transferred because they are associated with failed implantation, miscarriage, or a child born with a genetic condition.
A third category, mosaic embryos, presents a more complex interpretation. Mosaic embryos contain a mix of both normal and abnormal cells within the same embryo. The proportion of normal versus abnormal cells can vary, influencing the potential outcomes. While mosaicism has always existed, advanced PGS technologies have only recently made it detectable, leading to ongoing discussions about their transfer potential and associated risks.
There is a small possibility of false positives or false negatives, although the risk is low. A normal PGS result does not guarantee a live birth or a completely healthy baby, as the test does not detect all genetic issues or birth defects. Discussing the test results and their implications thoroughly with fertility specialists or genetic counselors is important to understand the specific situation and make informed decisions.