Preimplantation Genetic Testing (PGT) has transformed the landscape of assisted reproductive technology (ART) by allowing embryologists and fertility specialists to screen embryos for chromosomal abnormalities before implantation. The process, typically performed on day 5 embryos (blastocysts), aims to improve pregnancy outcomes and reduce the risk of miscarriage. However, an important question remains: how accurate is PGT in diagnosing chromosomal abnormalities in day 5 embryos?

Understanding Preimplantation Genetic Testing (PGT)

PGT is an umbrella term that includes different types of genetic testing, such as:

  • PGT-A (Aneuploidy Screening): Identifies numerical chromosomal abnormalities, ensuring only embryos with the correct number of chromosomes are transferred.
  • PGT-M (Monogenic Disease Testing): Screens for specific inherited genetic disorders, such as cystic fibrosis or sickle cell disease.
  • PGT-SR (Structural Rearrangements): Detects chromosomal structural abnormalities like translocations and inversions.

When PGT-A is performed, a small sample of cells is taken from the trophectoderm of the blastocyst (outer layer), as this biopsy is considered less invasive to the embryo’s inner cell mass, which develops into the fetus. The extracted DNA is then analyzed using different analytic techniques.

The Accuracy of PGT in Detecting Chromosomal Abnormalities

Sensitivity and Specificity of PGT-A
The sensitivity (ability to detect abnormalities when present) and specificity (ability to confirm normality when the embryo is truly normal) of PGT-A are generally high. Studies suggest that the accuracy of PGT-A exceeds 95% for detecting whole chromosomal aneuploidies. However, its accuracy can be influenced by various factors:

  • Mosaicism: Embryonic mosaicism occurs when some cells in the embryo have chromosomal abnormalities while others do not. Since PGT-A tests only a small subset of trophectoderm cells, it may not always reflect the chromosomal status of the entire embryo.
  • Biopsy Timing: Performing the biopsy on day 5 versus earlier stages (e.g., day 3) generally yields more accurate results because the blastocyst has undergone more cell divisions, allowing for a better representation of chromosomal status.
  • Technological Limitations: While NGS provides high resolution, it may still have a small margin of error in detecting partial deletions, duplications, or low-level mosaicism.

Understanding Mosaicism

Mosaicism refers to the presence of both normal and abnormal cells within the same embryo. Because PGT-A analyzes only a small sample of trophectoderm cells (typically 5-10), it is possible that some cells tested may be abnormal while others are normal. This limited sampling can lead to challenges in accurately diagnosing the embryo’s true chromosomal status.

The hope is that during further development, abnormal cells will stop growing while healthy cells continue to divide, leading to a healthy pregnancy. However, this self-correction is not guaranteed, and the level of mosaicism detected can influence clinical decisions. Testing a larger number of cells could improve accuracy, but this would come at the cost of potentially harming the embryo, making it less viable for implantation.

How Common is Mosaicism, and How Does It Affect Accuracy?

Mosaicism is detected in approximately 20% of embryos tested with PGT-A. Some embryos are classified as “low-level mosaic” (20-40% abnormal cells), while others are “high-level mosaic” (40-80% abnormal cells). The presence of mosaicism can lead to false positives (embryos classified as abnormal but capable of developing into healthy pregnancies) or false negatives (abnormal embryos mistakenly classified as normal). Despite advancements, PGT cannot always determine the developmental potential of mosaic embryos with absolute certainty.

Success Rates with Low Mosaic Embryos

Research has shown that low-level mosaic embryos (20-40% abnormal cells) can still lead to successful pregnancies and healthy live births. Studies indicate that implantation rates for low mosaic embryos range from 40-50%, which is lower than the 60-70% success rate observed with fully euploid embryos but still significant. Additionally, miscarriage rates with low mosaic embryos are slightly higher than with euploid embryos but remain considerably lower than with high mosaic or aneuploid embryos. Some fertility clinics now consider the transfer of low mosaic embryos as an alternative when no euploid embryos are available, as evidence suggests they have the potential to self-correct or develop normally. However, some low mosaics are not transferred, for example, when the abnormality detected could lead to a live birth with a condition such as Down’s syndrome (Trisomy 21).

False Positives and False Negatives in PGT-A

False positives occur when an embryo is labeled as abnormal due to a chromosomal aberration that may be confined to the trophectoderm and not affect the inner cell mass. This can lead to viable embryos being discarded. Conversely, false negatives happen when an embryo with an undetected chromosomal abnormality is implanted, potentially leading to implantation failure or miscarriage.

Studies suggest that false positive rates can range from 5% to 10%, while false negative rates remain lower but are still a consideration. This is why some clinics may consider transferring mosaic embryos under certain conditions, as some have been shown to result in healthy live births.

The Risk of a No-Reading Result

A no-reading result occurs when the PGT-A test is unable to provide a clear chromosomal analysis due to insufficient or degraded DNA from the sampled trophectoderm cells. This can happen for several reasons:

  • Poor DNA Quality: The extracted DNA may be fragmented or insufficient for analysis.
  • Technical Errors: Laboratory issues such as amplification failures or sequencing errors can lead to inconclusive results.
  • Low Cell Number: If too few cells are biopsied, the DNA content may be inadequate for a reliable reading.

When a no-reading result occurs, patients are often left with difficult choices. They may opt to rebiopsy the embryo (if feasible), discard it, or transfer it without chromosomal information, increasing uncertainty about implantation success and miscarriage risk. The incidence of no-reading results varies between laboratories but is generally estimated to occur in approximately 2-5% of embryos tested. While relatively uncommon, no-reading results highlight the limitations of current PGT technology and the importance of continued advancements in testing methodologies.

Clinical Implications of PGT Accuracy

Improving IVF Success Rates
By selecting euploid (chromosomally normal) embryos, PGT-A helps improve the chances of successful implantation and live birth rates. Studies indicate that euploid embryos selected through PGT-A have implantation rates of around 60-70%, compared to 30-40% for untested embryos.

Women over 35 experience a sharp increase in chromosomal abnormalities due to age-related declines in egg quality, making PGT-A particularly useful in improving pregnancy outcomes and reducing miscarriage rates in this group. For example, in a 42-year-old, the pregnancy rate is less than 10% with the transfer of an untested embryo, while a euploid embryo would have a 4-5 times higher chance due to the avoidance of embryos that would have resulted in a miscarriage. In contrast, for women under 35, where aneuploidy rates are much lower, PGT-A does not result in a statistically significant improvement in pregnancy rates compared to selecting embryos based on morphology alone. As a result, the utility of PGT-A in younger women remains a topic of ongoing discussion.

Reducing Miscarriage Risk
Aneuploidy is a leading cause of miscarriage, particularly in women of advanced maternal age. PGT-A significantly reduces the likelihood of transferring aneuploid embryos, thereby decreasing miscarriage rates and the emotional toll on patients.

Ethical Considerations and Controversies
Despite its benefits, PGT remains controversial. The potential discarding of embryos classified as abnormal or mosaic raises ethical concerns, especially since some mosaic embryos can develop into healthy babies. Additionally, the cost of PGT can be prohibitive, making it less accessible to all patients.

Conclusion

PGT is a highly accurate tool for diagnosing chromosomal abnormalities in day 5 embryos, but it is far from infallible.