PGS/PGT Testing: Complete Guide to Genetic Embryo Screening

Understand preimplantation genetic testing (PGT-A, PGT-M, PGT-SR). Learn when testing is recommended, what it detects, and how to interpret results.

Preimplantation Genetic Testing (PGT) represents one of the most significant advances in reproductive medicine, allowing embryos to be screened for chromosomal abnormalities or specific genetic conditions before transfer. While PGT can significantly reduce miscarriage rates and help select the healthiest embryos, it's not right for everyone. This guide explains the different types of PGT, who benefits most, and what to expect from the testing process.

Understanding the Different Types of PGT

PGT encompasses three distinct tests, each designed for different purposes. Understanding which applies to your situation helps you make informed decisions about testing.

  • PGT-A (Aneuploidy): Screens for chromosome number abnormalities—most common use
  • PGT-M (Monogenic): Tests for specific inherited single-gene disorders like cystic fibrosis or sickle cell
  • PGT-SR (Structural Rearrangements): For patients with known chromosomal translocations or inversions

You may see older terms like 'PGS' (Preimplantation Genetic Screening) or 'PGD' (Preimplantation Genetic Diagnosis). PGS is now called PGT-A, and PGD is now PGT-M.

PGT-A: Screening for Chromosomal Abnormalities

PGT-A is the most commonly used form of genetic testing in IVF. It screens embryos for aneuploidy—having too many or too few chromosomes. Aneuploidy is the leading cause of miscarriage and failed implantation, and becomes more common with maternal age.

  • Normal embryos have 46 chromosomes (23 pairs)—called 'euploid'
  • Abnormal chromosome numbers are called 'aneuploid'
  • Common aneuploidies: Trisomy 21 (Down syndrome), Monosomy X (Turner syndrome)
  • Most aneuploid embryos fail to implant or miscarry naturally
  • PGT-A identifies euploid embryos for transfer

The Biopsy Process

PGT requires removing a few cells from each embryo for testing. This biopsy is typically performed at the blastocyst stage (day 5-6) when the embryo has 100-200 cells. The biopsy removes 5-8 cells from the trophectoderm (cells that become placenta), leaving the inner cell mass (cells that become baby) untouched.

  • Biopsy is performed by a skilled embryologist under high magnification
  • Only trophectoderm cells are removed—ICM is not touched
  • Embryos are vitrified immediately after biopsy
  • Cells are sent to a specialized genetics laboratory
  • Results typically return in 1-2 weeks

The biopsy process is not completely without risk. Modern techniques have excellent survival rates (95-99%), but a small percentage of embryos may not survive the procedure.

Understanding Your PGT-A Results

Results are reported as Euploid (normal chromosome count), Aneuploid (abnormal chromosome count), or Mosaic (mix of normal and abnormal cells). Some results may come back as 'no result' due to insufficient DNA or technical issues.

  • Euploid: Normal 46 chromosomes—suitable for transfer
  • Aneuploid: Abnormal chromosome count—typically not transferred
  • Low-level Mosaic: Minor abnormalities—may be considered for transfer with counseling
  • High-level Mosaic: Significant abnormalities—rarely transferred
  • No Result: Technical issue—embryo may be re-biopsied

Who Benefits Most from PGT-A

While PGT-A can benefit many patients, research shows the greatest advantages for specific groups. The decision should consider your individual circumstances, embryo count, and goals.

  • Women 37+ years: Higher aneuploidy rates make screening more valuable
  • Recurrent miscarriage: PGT-A can identify if chromosomal issues are contributing
  • Recurrent implantation failure: May reveal undetected chromosomal problems
  • Previous aneuploid pregnancy: Higher risk of recurrence
  • Single embryo transfer preference: Helps select best embryo

For younger women with multiple embryos, the benefit of PGT-A is less clear. The test adds cost and embryos must be frozen while awaiting results. Discuss your specific situation with your doctor.

Limitations and Considerations

PGT-A is a powerful tool but has important limitations. It screens for chromosome number only—not every possible genetic condition. Results from trophectoderm biopsy may not perfectly reflect the inner cell mass. The test adds cost ($3,000-6,000) and requires a frozen embryo transfer.

  • False positives can occur: A euploid biopsy doesn't guarantee a euploid ICM
  • False negatives are rare but possible
  • Mosaicism adds complexity to decision-making
  • Some embryos may be discarded that could have self-corrected
  • Cost-effectiveness depends on individual circumstances

Key takeaways

  • PGT-A screens for chromosome number abnormalities (aneuploidy)
  • PGT-M tests for specific inherited genetic diseases
  • PGT-SR is for patients with known chromosomal rearrangements
  • Biopsy is performed on trophectoderm cells at blastocyst stage
  • Results categorize embryos as Euploid, Aneuploid, or Mosaic
  • Greatest benefit for women 37+, recurrent loss, or failed transfers
  • PGT-A reduces miscarriage risk but doesn't guarantee success
  • Results take 1-2 weeks; embryos are frozen during this time

Frequently asked questions

Does PGT-A improve live birth rates?

PGT-A improves success per transfer by selecting chromosomally normal embryos, but it doesn't create more normal embryos than you would have had. For younger women with many embryos, the cumulative success rate may be similar with or without testing. The benefit is clearer for older women and those with recurrent loss.

What if all my embryos are abnormal?

This can happen, especially with maternal age. Remember that without PGT-A, these embryos would have been transferred and likely resulted in failed implantation or miscarriage. This information, while devastating, helps you and your doctor plan next steps.

Should I transfer a mosaic embryo?

This is a nuanced decision requiring genetic counseling. Low-level mosaicism for certain chromosomes may be considered for transfer when no euploid embryos are available. Many healthy babies have been born from mosaic embryo transfers, but additional monitoring is typically recommended.

Can PGT-A detect all genetic problems?

No. PGT-A screens for chromosome number only. It cannot detect single-gene disorders, small deletions or duplications, or most genetic conditions. For specific inherited conditions, PGT-M is required and must be set up in advance of the IVF cycle.