PGT-A Testing Results: Euploid, Aneuploid, Mosaic

Your embryologist just emailed the report. You have a column of embryos with PGT-A testing results next to them: euploid, aneuploid, mosaic, maybe a "no result", and the page reads more like a chemistry exam than a pregnancy plan. You want to know what each row actually means before you walk into the transfer consult.

A PGT-A report is not a verdict on an embryo's potential. It is a five-cell trophectoderm biopsy interpreted through a probabilistic lens, and the difference between a clear decision and a panic over a normal-looking report is understanding what each category means and where the test's resolution ends. PGT-A testing results sort embryos into three primary categories (euploid, aneuploid, mosaic) plus a handful of less-common outcomes. This post walks through each row, what the numbers behind them mean, and where the test is silent.

I tell patients PGT-A narrows the field. It does not name the winner. A euploid transfer still has a real failure rate. An aneuploid almost never works. The mosaic question is its own conversation and has its own post. What this post does is get you fluent in the lab report so you can read it the way your RE reads it.

What PGT-A actually tests

On day 5, 6, or sometimes day 7 of culture, an embryologist removes about five cells from the trophectoderm of each blastocyst, the outer layer of cells that becomes placenta. The inner cell mass, the dense cluster that becomes the fetus, is not biopsied. DNA from those five cells is amplified and analyzed by next-generation sequencing (NGS), which reports chromosome count for each of the 23 chromosome pairs.

Two things follow from the biology. First, the test is sampling placenta-destined cells, not baby-destined cells, and the two lineages can diverge during embryonic development. Second, five cells is a small sample. The test is accurate at detecting whole-chromosome gains and losses, less so at detecting low-level mixed signals from a handful of cells. Both limits matter when you read mosaic and "no result" rows.

PGT-A testing results: the three main categories

Euploid

A euploid embryo has 46 chromosomes, appropriately XX or XY, with no aneuploidies detected. This is the result you are hoping for and the result that drives the standard transfer recommendation.

Implantation rates from registry data run roughly 55 to 65 percent per single euploid frozen embryo transfer in patients under 38, with live birth rates per transfer in the 50 to 60 percent range². Donor-egg euploid transfers run slightly higher. These numbers are higher than untested transfers of the same morphology grade, which is the per-transfer benefit PGT-A delivers. A euploid embryo is the best statistical bet you have. It is not a guarantee. A meaningful minority of euploid transfers do not implant, and a smaller share implant and then miscarry. Endometrial factors, immune factors, and unmeasured genetic factors all contribute to the gap between "euploid" and "live birth."

Aneuploid

An aneuploid embryo has one or more whole-chromosome gains or losses. Trisomy 16 is the most common aneuploidy in early embryos and the most common cause of first-trimester miscarriage. Trisomy 13, 18, and 21 are also seen. Monosomies (missing one of a chromosome pair) appear as well, with monosomy X (Turner syndrome) the only one viable past first trimester in some cases.

The standard recommendation for aneuploid embryos is to discard or not transfer. Most aneuploidies are incompatible with ongoing pregnancy and the small fraction that are compatible carry severe conditions. Some patients with trisomy 21 (Down syndrome) embryos elect transfer with full counseling. That is a deeply individual decision and should happen with a genetic counselor and the RE together, never on the lab report alone.

Mosaic

A mosaic result means the biopsy returned a mix of normal and abnormal cells. The report will list a percentage (for example, "low-level mosaic 30 percent" or "high-level mosaic 60 percent") and the chromosome involved. The percentage refers to the share of cells in the biopsy with the abnormality.

Mosaic results are reported in categories that vary slightly by lab. A common scheme is low-level mosaic (roughly 20 to 50 percent abnormal cells) and high-level mosaic (roughly 50 to 80 percent). Two further distinctions matter. Whether the abnormality is whole-chromosome (the entire chromosome involved) or segmental (only part of a chromosome): segmental mosaics generally carry lower risk. And whether the mosaic involves a chromosome that is viable to term, such as 13, 18, or 21, or one that almost always results in early miscarriage.

Outcomes from mosaic transfers in the published non-selection trial by Capalbo and colleagues in 2021 are surprisingly favorable for low-level mosaics, with live birth rates approaching those of euploid transfers in that cohort³. High-level mosaics performed less well but still produced live births. Importantly, follow-up of children born from mosaic transfers has shown the vast majority to be chromosomally normal at birth: the abnormal cells in the trophectoderm biopsy were apparently displaced from the fetus during development.

This is the genuinely hard category, and we have a dedicated post on whether to transfer a mosaic embryo because the decision deserves its own walkthrough.

Less common result categories

You may also see one of the following rows on a report.

• No result or inconclusive: The biopsy did not produce enough DNA, the sample failed amplification, or the result was technically ambiguous. Options are re-biopsy at additional cost (with another freeze-thaw cycle for the embryo) or transfer as untested. Both can be reasonable.
• Complex aneuploid: Multiple chromosomes affected. Not transferred.
• Segmental aneuploid: A partial chromosome gain or loss. Some segmentals can result in live birth; the data are limited and depend on the specific segment. Genetic counseling is appropriate before any transfer decision.

How age changes the euploid rate

If you are planning how many embryos to bank, this is the single most useful dataset in the literature. The Franasiak study published in Fertility and Sterility in 2014 reviewed 15,169 consecutive trophectoderm biopsies with comprehensive chromosomal screening and reported the proportion of biopsied blastocysts that came back euploid by maternal age¹.

• Under 35: roughly 60 to 65 percent euploid
• 35 to 37: roughly 50 percent euploid
• 38 to 40: roughly 30 to 40 percent euploid
• 41 to 42: roughly 15 to 25 percent euploid
• Over 42: roughly 5 to 10 percent euploid

These numbers explain why "how many embryos do I need to bank for one euploid" varies so much by age. A 33-year-old with four blastocysts has high odds of two or three euploids. A 41-year-old with four blastocysts may have one euploid, or none. Plan retrievals accordingly, and ask your RE about cumulative euploid yield rather than blast count alone.

What PGT-A does and does not predict

I want this in plain language because the marketing often blurs it.

PGT-A does dramatically reduce miscarriage rate per transfer. Untested transfers carry a roughly 25 percent miscarriage rate driven largely by aneuploid embryos slipping through morphology selection. Euploid transfers carry roughly 10 percent miscarriage rate, with the remaining losses driven by factors PGT-A cannot detect.

PGT-A does shorten time-to-pregnancy by selecting against the embryos that would have failed early.

PGT-A does not, on the published evidence, increase cumulative live birth rate per cycle started for general good-prognosis IVF patients. The STAR trial published by Munné and colleagues in 2019 found no cumulative live-birth advantage in the intention-to-treat analysis². The Yan trial in 2021 reached the same conclusion in a Chinese cohort of women aged 20 to 37. The cumulative-birth gap is the contested point. Per-transfer benefits are real; cumulative-cycle benefits are not consistent across populations.

PGT-A does not detect monogenic disease (that is PGT-M), structural rearrangements (that is PGT-SR), or anything outside chromosome count. PGT-A does not detect epigenetic problems, mitochondrial issues, or implantation-factor problems. A euploid embryo failing to implant tells you something the chromosome test cannot.

The mosaic question, briefly

About five to ten percent of biopsied blastocysts return mosaic results⁴. The PGDIS 2021 position statement on the transfer of mosaic embryos and the Capalbo non-selection trial both support transferring mosaics in carefully counseled patients, particularly when no euploid embryos are available and the chromosome involved is not high-risk³⁴. ASRM's committee opinion provides a clinical-management framework⁵.

I cover the decision tree in detail in the next post in this section. The short version: chromosome involved, percentage reported, whether you have euploid alternatives, and your tolerance for additional prenatal diagnostic testing during pregnancy all factor into whether transfer is the right call.

The cost-benefit conversation

PGT-A platform fees run USD 3,000 to 6,000 plus USD 200 to 500 per embryo biopsied. For some patients the math favors testing; for others it does not.

Best return on investment: advanced maternal age, recurrent pregnancy loss, recurrent implantation failure, very limited embryo cohorts where you cannot afford weeks of preparation for a transfer that has poor odds.

Lower return on investment: young good-prognosis patients with multiple blastocysts (the STAR and Yan trial populations), and any situation where the PGT cost would limit cycle number.

Counseling and genetic considerations

Pre-test genetic counseling is recommended in most centers, especially if PGT-M is being run alongside PGT-A or if you have a personal or family history that might be relevant. "No-result" embryos are not necessarily failing embryos. Re-biopsy or untested transfer are both reasonable on a case-by-case basis, and the decision should be made with the team, not made by the report.

Sex chromosome disclosure varies by clinic and by region. Most US clinics share by default; some on request only. Decide in advance how you want this information delivered and tell the team before the report is generated.

What to ask before reviewing your PGT-A report with your RE

A short list to bring to the consult.

• How many embryos were biopsied, and how many produced results?
• What is the breakdown across euploid, aneuploid, mosaic, and no result?
• For any mosaic, what is the percentage and which chromosome is involved? Is it whole-chromosome or segmental?
• What is your clinic's mosaic transfer policy and outcome data?
• Do you re-biopsy no-result embryos, transfer them as untested, or recommend discarding?
• Given my age and embryo cohort, what is your ranking for transfer order?
• What does grade contribute once ploidy is known?

What this means for your transfer plan

Your PGT-A testing results gave you an ordering of probabilities, not a guarantee. The euploid embryos are your best bets, in roughly the order of grade if you have multiple. Aneuploid embryos are off the table. Mosaic embryos are a separate conversation. A "no result" is not necessarily a "no go." Talk through your specific case with your clinic.

If you have a euploid embryo in the freezer and you are heading into transfer prep, the next post on embryo grading walks through how grade and ploidy interact when the team picks transfer order. If your report came back with a mosaic and you are weighing the call, head to the mosaic post next.

What's next

• If you have a euploid embryo ready for FET and want to understand transfer selection: embryo-grading-explained
• If your report includes a mosaic embryo and you are weighing transfer: should-transfer-mosaic-embryo
• If you want the broader PGT context first: pgt-a-m-sr-explained
• If a euploid transfer has already failed and you are working through next steps: failed-ivf-decoding-next

Sources

1. Franasiak JM, Forman EJ, Hong KH, et al. 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 2014;101(3):656-663.e1. https://doi.org/10.1016/j.fertnstert.2013.11.004
2. Munné S, Kaplan B, Frattarelli JL, et al. Preimplantation genetic testing for aneuploidy versus morphology as selection criteria for single frozen-thawed embryo transfer in good-prognosis patients: a multicenter randomized clinical trial. Fertility and Sterility 2019;112(6):1071-1079.e7. https://doi.org/10.1016/j.fertnstert.2019.07.1346
3. Capalbo A, Poli M, Rienzi L, et al. Mosaic human preimplantation embryos and their developmental potential in a prospective, non-selection clinical trial. American Journal of Human Genetics 2021;108(12):2238-2247. https://doi.org/10.1016/j.ajhg.2021.11.002
4. Preimplantation Genetic Diagnosis International Society (PGDIS). PGDIS Position Statement on the Transfer of Mosaic Embryos 2021. https://www.pgdis.org/
5. Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. The use of preimplantation genetic testing for aneuploidy: a committee opinion. Fertility and Sterility 2018;109(3):429-436. https://doi.org/10.1016/j.fertnstert.2018.01.002
6. ESHRE PGT Consortium and SIG Embryology. ESHRE PGT Consortium good practice recommendations for the detection of structural and numerical chromosomal aberrations. Human Reproduction Open 2020;2020(3):hoaa017. https://doi.org/10.1093/hropen/hoaa017