Preimplantation Genetic Testing for Aneuploidy (PGT-A) is a common procedure used during In Vitro Fertilization (IVF) cycles. This genetic screening method helps identify embryos with the correct number of chromosomes before they are transferred to the uterus. The primary goal of PGT-A is to improve the chances of a successful pregnancy and reduce the risk of miscarriage by selecting the most viable embryos.
Understanding Preimplantation Genetic Testing
PGT-A involves a sophisticated process where a small number of cells are carefully removed from an embryo. This biopsy typically occurs when the embryo has developed to the blastocyst stage, usually on day 5 or 6 after fertilization. The biopsied cells are then sent for genetic analysis.
The analysis specifically looks for aneuploidy, which refers to an abnormal number of chromosomes within the cells. Human cells normally contain 46 chromosomes arranged in 23 pairs. Aneuploidy can involve missing or extra chromosomes, such as having three copies of chromosome 21 (Trisomy 21, or Down syndrome) instead of the usual two. The purpose of PGT-A is to identify euploid embryos, which possess the correct number of chromosomes, making them more likely to implant successfully and develop into a healthy pregnancy.
The Percentage of Embryos That Pass
The percentage of embryos that “pass” PGT-A testing varies significantly among individuals. On average, studies indicate that approximately 30% to 60% of embryos tested through PGT-A are found to be euploid. This range reflects diverse biological factors and individual circumstances.
An embryo “passing” PGT-A means it has been identified as euploid, containing the normal complement of 46 chromosomes. These euploid embryos are generally prioritized for transfer due to their higher potential for successful implantation and a healthy live birth. Conversely, embryos identified as aneuploid contain an abnormal number of chromosomes and are typically not recommended for transfer because they often lead to implantation failure, miscarriage, or the birth of a child with a genetic condition.
Some embryos may also be classified as mosaic, meaning they contain a mixture of both euploid and aneuploid cells. The proportion of normal versus abnormal cells in mosaic embryos can vary, influencing their potential viability. While mosaic embryos are generally considered to have a lower implantation potential compared to euploid embryos, their transfer may be considered in specific clinical situations, especially when no euploid embryos are available. Decisions regarding mosaic embryo transfer are complex and require thorough discussion with a fertility specialist.
Factors Affecting PGT-A Outcomes
A primary factor influencing the percentage of euploid embryos obtained from an IVF cycle is the maternal age. As a woman ages, particularly after the mid-30s, the likelihood of producing eggs with chromosomal abnormalities increases. This age-related decline in egg quality is a significant contributor to lower euploidy rates and reduced IVF success rates.
Beyond maternal age, the initial quality and developmental progress of the embryos also play a role. Embryos that develop well to the blastocyst stage and exhibit good morphological grading are generally more likely to be euploid. While less impactful than maternal age, the specific protocols and laboratory conditions within an IVF clinic can also subtly influence embryo development and, consequently, the proportion of embryos reaching the blastocyst stage suitable for biopsy. However, the biological factors inherent to the gametes remain the most dominant determinants of euploidy rates.
What PGT-A Results Mean for Your Journey
PGT-A results provide valuable information that guides critical decisions in an IVF journey. Euploid embryos are prioritized for transfer, offering the highest chance of a successful pregnancy and reduced miscarriage risk. This approach optimizes IVF efficiency by avoiding transfers of chromosomally abnormal embryos.
Aneuploid embryos are typically not transferred due to high risk of negative outcomes. If only mosaic embryos are available, their transfer may be considered, but this requires careful counseling about potential risks and reduced success rates. PGT-A results can ultimately help reduce the number of embryo transfers needed to achieve a pregnancy, potentially shortening the overall time to pregnancy and minimizing the emotional and financial burden of repeated failed cycles.