What is PGT-A? A Patient's Guide to Preimplantation Genetic Testing for Aneuploidy

Your IVF clinic mentions PGT-A. You see it listed on your treatment plan as an optional test. Your doctor says it "screens embryos for chromosome problems." But you're left with questions: What exactly does that mean? Is it worth the cost? Will it guarantee success?

PGT-A (Preimplantation Genetic Testing for Aneuploidy) is a genetic screening test performed on embryos created during an IVF cycle. Its purpose is to identify embryos with the correct number of chromosomes (46 in humans), giving you the best chance of a healthy pregnancy. By selecting a chromosomally normal (euploid) embryo for transfer, PGT-A can increase pregnancy rates per transfer by 10-20 percentage points, can reduce miscarriage risk, and lower the chance of certain chromosome abnormalities like Down syndrome (though standard prenatal testing is still recommended).[1]

Here's the critical context most patients don't know: Even in young, healthy couples, 30-50% of embryos created during IVF are chromosomally abnormal (aneuploid). In women over 40, that number rises to 70-80%.[2] These aneuploid embryos almost never result in live births—they either fail to implant or end in early miscarriage. PGT-A helps you avoid transferring these embryos, saving time, emotional energy, and reducing the physical toll of repeated cycles.

But here's what PGT-A is not: It's not a guarantee of pregnancy. It's not a diagnostic test (it's a screening test with a small margin of error). And it doesn't check for single-gene disorders like cystic fibrosis—that requires a different test called PGT-M.

This guide explains what aneuploidy is, who should consider PGT-A, how the process works, what your PGT-A result will tell you, and—crucially—what the test cannot do. By the end, you'll understand whether using PGT-A is right for your fertility journey.

Understanding the "A" in PGT-A: What is Aneuploidy?

To understand PGT-A, you first need to know what aneuploidy means.

Chromosomes are the structures that carry your genetic material—the DNA instructions for building and running a human body. Normally, humans have 46 chromosomes arranged in 23 pairs: one set of 23 from the egg, one set from the sperm.

Euploidy: The correct number of chromosomes (46 total). This is what we want in an embryo.

Aneuploidy: An incorrect number of chromosomes—either too many or too few. Common examples:

Trisomy 21 (Down syndrome): 3 copies of chromosome 21 instead of 2 (47 total chromosomes)
Monosomy X (Turner syndrome): Only 1 sex chromosome instead of 2 (45 total chromosomes)
Trisomy 16: 3 copies of chromosome 16 (incompatible with life; causes early miscarriage)

Why does aneuploidy happen?

Most aneuploidy occurs during egg or sperm formation, when chromosomes fail to separate properly (called "nondisjunction"). This is a random error that increases dramatically with maternal age. As women age, the cellular machinery that divides chromosomes becomes less precise.[3]

Aneuploidy is the leading cause of:

Implantation failure (70% of failed IVF cycles)[4]
Early miscarriage (60-70% of first-trimester losses)[5]

Some live births with chromosome disorders (like Down syndrome, which occurs in 1 in 700 births)

Aneuploidy Risk by Maternal Age

Maternal Age	Aneuploidy Rate in Embryos
Under 30	30-40%
30-34	35-45%
35-37	40-50%
38-40	50-60%
41-42	60-70%
43+	75-85%

(Data from Franasiak et al., 2014)[2]

Key takeaway: Aneuploidy is not caused by anything you did or didn't do. It's a biological reality of human reproduction. PGT-A doesn't prevent aneuploidy—it identifies which embryos are affected so you can transfer a euploid one.

Who Should Consider PGT-A?

PGT-A is not medically necessary for everyone undergoing IVF, but it offers significant benefits for certain groups. The American Society for Reproductive Medicine (ASRM) and most reproductive medicine specialists recommend considering PGT-A if you fall into any of these categories:[6]

✓ Women age 35 and older
As shown in the table above, aneuploidy rates rise sharply after 35. Using PGT-A helps identify the euploid embryos among a cohort where 50%+ may be aneuploid.

✓ History of recurrent miscarriage
If you've had 2+ early pregnancy losses, chromosome aneuploidy is a likely cause. PGT-A can help select an embryo less likely to miscarry.

✓ Multiple failed IVF cycles
If you've transferred multiple embryos without successful implantation, aneuploidy may be the culprit. PGT-A increases implantation rates by ensuring the embryo you transfer is chromosomally capable of developing.

✓ Previous pregnancy affected by chromosome abnormality
If you've had a pregnancy with trisomy or other abnormality, future embryos may also be at higher risk.

✓ Desire for single embryo transfer with confidence
Many patients want to transfer only one embryo to avoid multiples (twins/triplets), but worry about success rates. PGT-A allows confident single embryo transfer (eSET) by confirming chromosomal normalcy, maintaining high success rates while eliminating multiple pregnancy risk.[7]

✓ Severe male factor infertility
Some studies suggest higher aneuploidy rates in embryos created via ICSI with severe sperm abnormalities, though this is debated.

Who might NOT need PGT-A?

Women under 35 with no miscarriage history and good embryo quality
Patients with very few embryos (1-2 total) where testing all may leave no euploid embryos for transfer
Those who prefer to transfer without genetic testing for personal, religious, or ethical reasons

The decision to use PGT-A is personal. Discuss your family history, age, IVF history, and goals with your fertility specialist and genetic counselor.

The PGT-A Procedure: A Step-by-Step Guide

PGT-A is performed as part of your IVF cycle. Here's exactly what happens:

Step 1: Embryo Development (Days 1-5/6/7)

Your IVF cycle proceeds normally:

Ovarian stimulation → egg retrieval → fertilization (standard IVF or ICSI) → embryo culture

Embryos are grown in the lab for 5-7 days until they reach the blastocyst stage (100-200 cells with distinct structures). PGT-A is only performed on blastocysts, not earlier-stage embryos, because blastocysts have enough cells for biopsy without harming the embryo.[8]

Step 2: Embryo Biopsy (Day 5/6/7)

An embryologist performs a trophectoderm biopsy: 5-10 cells are carefully removed from the outer layer of the blastocyst (the trophectoderm, which will become the placenta). The inner cell mass (which becomes the baby) is not touched.

Is the biopsy safe? Yes. Decades of data show that trophectoderm biopsy does not harm the future baby or reduce pregnancy success rates.[9] The embryo has over 100 cells at this stage, and removing 5-10 from the placental layer does not affect the fetus.

Step 3: Embryo Freezing (Cryopreservation)

Immediately after biopsy, embryos are frozen (vitrified) using ultra-rapid freezing. This is necessary because PGT-A results take 1-2 weeks. All PGT-A cycles require frozen embryo transfer (FET).[10]

The embryos remain safely frozen while the biopsied cells are sent to a genetic testing laboratory for analysis.

Step 4: Chromosomal Analysis (1-2 Weeks)

The lab extracts DNA from the biopsied cells and uses advanced genetic test technology (typically Next-Generation Sequencing, or NGS) to count the chromosomes in each embryo.

The lab checks all 23 chromosome pairs (22 autosomes plus the sex chromosomes X and Y) to determine if each embryo is:

Euploid (normal: 46 chromosomes)
Aneuploid (abnormal: extra or missing chromosomes)
Mosaic (a mix of normal and abnormal cells—more on this below)

Step 5: Reviewing Results and Selecting an Embryo for Transfer

Your fertility clinic receives the PGT-A report. You meet with your doctor (and often a genetic counselor) to review the findings. Based on the PGT results, you select the best embryo (euploid) for transfer.

The euploid embryo is thawed and transferred in a subsequent cycle (usually 1-2 months after retrieval).

Understanding Your PGT-A Results

Your PGT-A result will classify each embryo into one of several categories. Here's what each means:

Results Explained Table

Result	What It Means	Number of Chromosomes	Recommended Action
Euploid	Chromosomally normal	46 (23 pairs)	High priority for transfer. Best chance of implantation and healthy pregnancy.
Aneuploid	Chromosomally abnormal	45, 47, or other	Not recommended for transfer. Very low chance of pregnancy; high miscarriage risk.
Mosaic	Mix of normal and abnormal cells	Varies	Requires in-depth discussion. May be considered for transfer in some cases (see below).
Segmental Mosaic	Partial chromosome deletion/duplication in some cells	Varies	Complex case. Discuss with genetic counselor. Transfer considered case-by-case.
No Result	Insufficient or poor-quality sample	Unknown	May require re-biopsy or exclude from PGT process.

Euploid Embryos: Your Best Candidates

An euploid embryo has the correct number of chromosomes (46). This is the ideal result. Euploid embryos have:[1][7]

60-70% chance of implantation per transfer (compared to 30-40% without PGT-A)
Significantly lower miscarriage rates (8-10% vs 20-30% for unscreened embryos)
Very low risk of chromosome abnormalities like Down syndrome (though not zero—see limitations section)

If you have multiple euploid embryos, your doctor will prioritize transfer based on embryo morphology (grade) and possibly your personal preferences (e.g., if you want to know the sex).

Aneuploid Embryos: Not Recommended for Transfer

Aneuploid embryos have an incorrect number of chromosomes. Common aneuploidies seen in PGT-A reports:

Trisomy 15, 16, 21, 22 (3 copies instead of 2)
Monosomy X (only 1 X chromosome instead of 2)
Complex aneuploidy (multiple chromosome errors)

Why not transfer aneuploid embryos?
95%+ of aneuploid embryos will either:

Fail to implant
Result in early miscarriage (usually before 8-10 weeks)
In rare cases (like Trisomy 21), result in a live birth with a chromosome disorder[11]

Aneuploid embryos are typically not transferred. They remain frozen but are not used, or they may be discarded or donated to research (per your consent).

Mosaic Embryos: The Complex Middle Ground

A mosaic embryo is one where some cells are euploid and others are aneuploid. For example, 30% of cells have trisomy 16, while 70% are normal.

How does mosaicism happen?
Mosaicism occurs when chromosome errors arise during early embryo development (after fertilization), causing some cells to be abnormal while others remain normal. This is different from full aneuploidy, which starts at fertilization.[12]

Can mosaic embryos result in healthy babies?
Yes. Studies show that mosaic embryos can self-correct (the abnormal cells may die off or be excluded from the developing fetus) and result in healthy pregnancies in 30-50% of cases—lower than euploid (60-70%) but higher than aneuploid (<5%).[13][14]

When are mosaic embryos transferred?
If you have no euploid embryos available, your fertility specialist may recommend transferring a mosaic embryo after detailed genetic counseling. Factors considered:

Percentage of mosaicism: Low-level mosaicism (20-40%) is more favorable than high-level (60-80%)
Which chromosome is affected: Some chromosomes (like 16, 22) are less likely to cause problems than others (like 13, 18, 21)
Your age and number of embryos available
Your comfort level with uncertainty

Important: If a mosaic embryo results in pregnancy, comprehensive prenatal genetic testing (amniocentesis or CVS) is recommended to confirm the fetus is chromosomally normal.[15]

Segmental Mosaicism: Partial Chromosome Changes

Segmental mosaicism is a variant where some cells have a partial chromosome deletion or duplication (not a whole chromosome aneuploidy). For example, part of chromosome 7 is missing in 40% of cells.

Segmental mosaicism is even more complex than regular mosaicism. Transfer decisions are made case-by-case with extensive genetic counseling.[16]

No Result / Inconclusive

Occasionally, PGT-A labs cannot generate a result due to:

Poor sample quality (not enough DNA extracted)
Technical issues
Embryo degradation during biopsy

If this happens, the embryo may be re-biopsied (if it has enough cells), or it may be excluded from PGT and transferred based on morphology alone.

PGT-A vs. PGT-M vs. PGT-SR: What's the Difference?

PGT (preimplantation genetic testing) is an umbrella term. There are three types of PGT, each designed to detect different genetic conditions. Here's how they differ:

Test Type	What It Checks	Who Needs It
PGT-A (Aneuploidy)	Number of chromosomes (all 23 pairs)	Anyone wanting to reduce aneuploidy risk; especially women 35+, recurrent miscarriage, failed IVF
PGT-M (Monogenic/Single-Gene Disorders)	Specific genetic mutations (e.g., cystic fibrosis, sickle cell, BRCA)	Couples who are known carriers of a genetic disorder or have family history
PGT-SR (Structural Rearrangements)	Balanced translocations or chromosome rearrangements	Individuals with known chromosome rearrangements detected via karyotype testing

Key difference: PGT-A is broad screening (checking all chromosomes for quantity). PGT-M and PGT-SR are targeted tests for known issues.

Can you do PGT-A and PGT-M together?
Yes. If you're a carrier of a single-gene disorder (requiring PGT-M) and also at higher risk of aneuploidy (age 35+), PGT-A and PGT-M can be performed on the same biopsy sample. This is sometimes called "PGT-M with PGT-A add-on."[17]

Benefits and Limitations of PGT-A

The Advantages of PGT-A

Increased Pregnancy Rates Per Transfer
By transferring only euploid embryos, PGT-A improves implantation rates. Studies show pregnancy rates per embryo transfer increase by 10-20 percentage points compared to transferring unscreened embryos.[1][7]

Example: Without PGT-A, pregnancy rate per transfer: 40-50%. With PGT-A (euploid embryo only): 60-70%.

Significantly Reduced Miscarriage Rates
Aneuploidy causes 60-70% of first-trimester miscarriages. PGT-A eliminates most of these by avoiding aneuploid embryo transfers. Miscarriage rates drop from 20-30% (unscreened) to 8-10% (PGT-A screened).[5][18]
Decreased Time to Pregnancy
By avoiding failed transfers of aneuploid embryos, PGT-A reduces the number of IVF cycles needed to achieve pregnancy. For women over 38, this can save 6-12 months.[19]
Confidence in Single Embryo Transfer
PGT-A allows you to confidently transfer one embryo (reducing twin/triplet risk) while maintaining high success rates. Without PGT-A, clinics often transfer 2 embryos to improve odds, increasing multiple pregnancy risk.[7]
Reduced Chromosome Disorder Risk
Although PGT-A is not 100% accurate, it significantly reduces the chance of live birth with conditions like Down syndrome (Trisomy 21) or Edwards syndrome (Trisomy 18).[20]

What PGT-A Cannot Do (Limitations)

It's a Screening Test, Not a Diagnostic Test
PGT-A analyzes 5-10 cells from the trophectoderm. There's a small chance (<5%) of false positive (calling a normal PGT-A embryo aneuploid) or false negative (calling an aneuploid embryo euploid) due to mosaicism or technical error.[21]

This is why standard prenatal screening (NIPT, ultrasound, amniocentesis) is still recommended during pregnancy, even after PGT-A.

PGT-A Cannot Check for Single-Gene Disorders
PGT-A checks chromosome quantity, not gene mutations. It won't detect cystic fibrosis, Tay-Sachs, sickle cell, or other single-gene disorders. For those, you need PGT-M.[22]
It Doesn't Guarantee a Live Birth
Even euploid embryos have a 30-40% chance of not implanting or resulting in pregnancy loss due to factors unrelated to chromosomes: uterine issues, immune factors, blood clotting disorders, or unknown causes.

PGT-A may help, but it's not a guarantee.

It Doesn't Address All Pregnancy Risks
PGT-A reduces chromosome-related miscarriage, but it doesn't prevent other complications: preterm birth, placental issues, gestational diabetes, preeclampsia. Comprehensive prenatal care is essential.
Not All Mosaic Embryos Are Problematic
PGT-A may flag mosaic embryos as "abnormal," but many can self-correct and result in healthy babies. Discarding all mosaic embryos may reduce your chances if you have limited embryos.[13][14]
Cost and Emotional Burden
PGT-A adds $3,000-$5,000 to IVF costs (often not covered by insurance). It also requires frozen embryo transfer, extending your timeline by 1-2 months. And learning that most or all of your embryos are aneuploid can be emotionally devastating.

Conclusion: Making an Informed Decision with Your Doctor

PGT-A is a powerful tool that provides crucial information about your embryos' chromosomal health, helping you and your fertility team select the embryo with the highest chance of a successful, healthy pregnancy. For many patients—especially those over 35, with **recurrent miscarriage, or with failed IVF history—the benefit of PGT-A can be life-changing, reducing emotional and physical stress while improving outcomes.

But PGT-A is not magic. It's a screening test that increases your odds per transfer and reduces miscarriage risk, but it doesn't eliminate all uncertainty. Some euploid embryos don't implant. Some mosaic embryos do. Pregnancy is complex, and PGT-A addresses only one piece of the puzzle—chromosomes.

Key takeaways:

PGT-A screens embryos for the correct number of chromosomes (46), identifying euploid (normal) vs. aneuploid (abnormal) embryos.
Aneuploidy is extremely common—30-50% of all embryos in young women, 70-80% in women over 40.[2]
Using PGT-A may increase pregnancy rates per transfer by 10-20 percentage points and cut miscarriage rates in half.[1][18]
PGT-A is most beneficial for women 35+, those with miscarriage or failed IVF history, and anyone wanting confident single embryo transfer.
Mosaic results are complex and require detailed genetic counseling. They're not automatically "bad."
PGT-A also has limitations: it's a screening test (not diagnostic), doesn't check single-gene disorders, and doesn't guarantee pregnancy.

The decision to pursue PGT-A is deeply personal. It depends on your age, IVF history, number of embryos, risk tolerance, and financial situation. Talk with a genetic counselor and your fertility specialist to weigh the pros and cons for your unique case.

Remember: There is no "wrong" choice. Some patients do PGT-A and feel empowered by the information. Others prefer to transfer embryos based on morphology alone. Both approaches can lead to healthy babies.

Scientific References

[1] Rubio, C., Bellver, J., Rodrigo, L., et al. (2017). In vitro fertilization with preimplantation genetic diagnosis for aneuploidies in advanced maternal age: a randomized, controlled study. Fertility and Sterility, 107(5), 1122-1129.

[2] Franasiak, J. M., Forman, E. J., Hong, K. H., et al. (2014). The nature of aneuploidy with increasing age of the female partner: a review of 15,169 consecutive trophectoderm biopsies evaluated with comprehensive chromosomal screening. Fertility and Sterility, 101(3), 656-663.

[3] Nagaoka, S. I., Hassold, T. J., & Hunt, P. A. (2012). Human aneuploidy: mechanisms and new insights into an age-old problem. Nature Reviews Genetics, 13(7), 493-504.

[4] Garrisi, G. J., Colls, P., Ferry, K. M., et al. (2009). Effect of infertility, maternal age, and number of previous miscarriages on the outcome of preimplantation genetic diagnosis for idiopathic recurrent pregnancy loss. Fertility and Sterility, 92(1), 288-295.

[5] Practice Committee of the American Society for Reproductive Medicine. (2012). Evaluation and treatment of recurrent pregnancy loss. Fertility and Sterility, 98(5), 1103-1111.

[6] Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. (2018). The use of preimplantation genetic testing for aneuploidy (PGT-A): a committee opinion. Fertility and Sterility, 109(3), 429-436.

[7] Scott, R. T., Upham, K. M., Forman, E. J., et al. (2013). Cleavage-stage biopsy significantly impairs human embryonic implantation potential while blastocyst biopsy does not. Reproductive BioMedicine Online, 27(3), 323-329.

[8] Capalbo, A., Rienzi, L., Cimadomo, D., et al. (2014). Correlation between standard blastocyst morphology, euploidy and implantation. Reproductive BioMedicine Online, 28(3), 305-311.

[9] Tobler, K. J., Zhao, Y., Ross, R., et al. (2015). Blastocyst biopsy for preimplantation genetic diagnosis does not reduce live birth rates or increase miscarriage rates. Fertility and Sterility, 103(6), 1454-1460.

[10] Roque, M., Haahr, T., Geber, S., et al. (2019). **Fresh versus elective frozen embryo transfer in IVF/ICSI cycles: a systematic review and meta-analysis of reproductive outcomes. Human Reproduction Update, 25(1), 2-14.

[11] Hassold, T., & Hunt, P. (2001). To err (meiotically) is human: the genesis of human aneuploidy. Nature Reviews Genetics, 2(4), 280-291.

[12] Munné, S., Grifo, J., & Wells, D. (2016). Mosaicism: "survival of the fittest" versus "no embryo left behind." Fertility and Sterility, 105(5), 1146-1149.

[13] Greco, E., Minasi, M. G., & Fiorentino, F. (2015). Healthy babies after intrauterine transfer of mosaic aneuploid blastocysts. New England Journal of Medicine, 373(21), 2089-2090.

[14] Victor, A. R., Griffin, D. K., Brake, A. J., et al. (2019). Assessment of aneuploidy concordance between clinical trophectoderm biopsy and blastocyst. Human Reproduction, 34(1), 181-192.

[15] PGDIS Position Statement on Chromosome Mosaicism and Preimplantation Aneuploidy Testing at the Blastocyst Stage. (2016). Preimplantation Genetic Diagnosis International Society.

[16] Handyside, A. H., Montag, M., Magli, M. C., et al. (2012). Multiple meiotic errors caused by predivision of chromatids in women of advanced maternal age undergoing in vitro fertilisation. European Journal of Human Genetics, 20(7), 742-747.

[17] Thornhill, A. R., deDie-Smulders, C. E., Geraedts, J. P., et al. (2005). ESHRE PGD Consortium 'Best practice guidelines for clinical preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS).' Human Reproduction, 20(1), 35-48.

[18] Harton, G. L., Munné, S., Surrey, M., et al. (2013). Diminished effect of maternal age on implantation after preimplantation genetic diagnosis with array comparative genomic hybridization. Fertility and Sterility, 100(6), 1695-1703.

[19] Lee, E., Illingworth, P., Wilton, L., et al. (2009). The clinical effectiveness of preimplantation genetic diagnosis for aneuploidy in all 24 chromosomes (PGD-A). Human Reproduction, 24(10), 2419-2425.

[20] Hodes-Wertz, B., Grifo, J., Ghadir, S., et al. (2012). Idiopathic recurrent miscarriage is caused mostly by aneuploid embryos. Fertility and Sterility, 98(3), 675-680.

[21] Popovic, M., Dheedene, A., Christodoulou, C., et al. (2018). Chromosomal mosaicism in human blastocysts: the ultimate challenge of preimplantation genetic testing? Human Reproduction, 33(7), 1342-1354.

[22] Kuliev, A., Zlatopolsky, Z., Kirillova, I., et al. (2011). Preimplantation genetic diagnosis for chromosomal disorders. Reproductive BioMedicine Online, 22(4), 360-368.