The Only Blastocyst Has an Abnormal PGT-A Result: Could It Be a False Positive?

Understanding PGT-A requires distinguishing whole-chromosome aneuploidy, mosaic findings, segmental changes and an inconclusive result. The same phrase, “not recommended for transfer”, can conceal very different findings. This article examines what the test samples, what outcome studies show, and the physical and time costs relevant to decisions about a single remaining blastocyst.

A difficult situation in reproductive genetic counselling is having only one or two blastocysts after stimulation, retrieval, fertilization and culture, then receiving a report showing a missing or extra chromosome, several abnormalities, a mosaic percentage or “no result”. The suggested transfer status may look similar despite these differences.

Two understandable questions often follow:

These questions involve probability, developmental biology and clinical consequences. The answer depends on the exact result and the laboratory's methods, rather than the word “abnormal” alone.

01 An untested embryo and a known abnormal result

PGT-A does not improve an embryo's chromosome complement. Testing changes what is known about the embryo.

For a woman around age 35, roughly half of tested blastocysts may be euploid in some datasets; the proportion varies with age and the population studied.[11] Transfer without testing accepts uncertainty about chromosome status. A chromosomally abnormal embryo may fail to implant, lead to biochemical pregnancy or miscarriage, or, for some abnormalities, an affected ongoing pregnancy. A roughly 50% euploid proportion is not a 50% live-birth probability.

A clear nonmosaic whole-chromosome finding substantially changes that assessment. It is different from choosing among embryos whose chromosome status is unknown, although PGT-A is not an infallible diagnosis of every cell in the embryo.

A particularly informative design is a nonselection study: embryos are biopsied, the result is withheld, and transfer selection proceeds by morphology. Outcomes are assessed before the genetic results are disclosed. In the multicentre study by Tiegs and colleagues published in 2021, none of 102 embryos classified as aneuploid produced an ongoing pregnancy.[1] ASRM's 2024 review reports an estimated clinical diagnostic-error rate of 0%, with a 95% confidence interval of 0–2.43%, for those data.[2] That interval is not an individual's probability of a false positive or a successful transfer, and zero observed ongoing pregnancies does not prove universal impossibility.

Testing provides information for selection. A clearly abnormal result should therefore prompt interpretation of the report rather than an assumption that the embryo retains the same probability as an untested embryo.

02 What false positives mean in PGT-A

Reports of healthy births after transfer of embryos with abnormal PGT-A findings need to be read by result category. Mosaic, segmental and nonmosaic whole-chromosome findings do not carry identical evidence.

A blastocyst contains an outer trophectoderm, which contributes predominantly to the placenta, and an inner cell mass, which contributes to the fetus. Biopsy commonly samples around 5–10 outer cells to avoid sampling the inner cell mass. The report therefore describes the DNA signal from that sample. A discrepancy between the sampled cells and the rest of the embryo can occur.

Mosaic findings, usually labelled “mosaic” with a percentage, and some segmental deletions or duplications, labelled del or dup, are particularly relevant. Many arise from errors after fertilization, so different cell populations may carry different findings; technical factors can also influence the signal. A reported mosaic percentage is an estimate from pooled DNA, not a direct census of every abnormal cell in the embryo.

Nonmosaic whole-chromosome abnormalities, often written as “−” or “+” followed by a chromosome number, commonly arise during meiosis in the egg or sperm. They are consequently more likely to involve both outer and inner compartments. In Girardi and colleagues' study of 390 blastocysts with multiple trophectoderm samples and an inner-cell-mass sample, the positive predictive value was 97.18% for whole-chromosome and 70.8% for segmental abnormalities.[4] These are study-specific predictive values, not universal personal diagnostic accuracies.

Complex results involve several chromosomes, for example one deletion and another gain, or three or four abnormalities. Age-related meiotic errors may contribute, but multiple abnormal signals should be reviewed together; laboratory errors are not necessarily independent events, so their false-positive probabilities cannot simply be multiplied. If mosaic and nonmosaic findings coexist, a genetic counsellor should interpret each component and the overall profile.

Copy-number plots can help explain how a laboratory classified the sample:

Report categories and possible next steps

Laboratory mosaic thresholds differ; the same DNA signal can be classified differently by different laboratories. A clear nonmosaic whole-chromosome loss should therefore be distinguished from a borderline or segmental signal, while retaining the limitations of sampling.

03 Which embryos were involved in reports of healthy births?

Many widely cited reports involve mosaic findings rather than clear nonmosaic whole-chromosome abnormalities.

In Greco and colleagues' 2015 New England Journal of Medicine report, 18 women with no euploid embryos each received one mosaic blastocyst; six had healthy term births.[6] It established the importance of considering mosaic embryos as a distinct category.

Capalbo and colleagues' large blinded nonselection study reported live-birth rates of 43.4% for euploid embryos, 42.9% for low-range mosaic and 42.0% for moderate-range mosaic embryos.[3] This evidence concerned findings below 50% on that study's platform. Higher-range mosaic embryos were not transferred, and the results do not establish equivalent outcomes for all mosaic or whole-chromosome aneuploid embryos.

Other evidence differs. Viotti and colleagues' multicentre observational dataset of approximately 1,000 mosaic transfers found lower implantation and more than twice the miscarriage rate compared with euploid transfer, with better outcomes for lower-range or segmental mosaic findings.[5] Differences in classification, selection and populations help explain why a single mosaic success rate cannot be applied to everyone.

Barad and colleagues studied patients seeking transfer of embryos considered abnormal elsewhere: 50 patients underwent 57 transfer cycles, resulting in 19 clinical pregnancies, 11 miscarriages and eight live births. Mean age was approximately 41 years. One infant had a segmental duplication and required cardiac surgery.[7] This self-selected cohort does not represent all patients or all abnormal-result categories.

04 The physical and time costs of attempting transfer

Transfer failure can mean more than a negative pregnancy test. A whole-chromosome abnormal embryo can follow different paths:

• No implantation or biochemical pregnancy, particularly with many autosomal monosomies. This still entails endometrial preparation, transfer costs, luteal-support medication and waiting.
• Implantation followed by miscarriage, sometimes after a heartbeat has been seen. Chromosome abnormalities are a major cause of early miscarriage. In a study of 7,118 miscarriage specimens, 67.25% had an abnormal karyotype.[12]
• An ongoing affected pregnancy. Trisomies 13, 18 and 21 and some sex-chromosome abnormalities may survive to birth. Prenatal diagnosis and decisions about continuation of pregnancy may then become necessary.

Miscarriage can have several consequences:

• Physical recovery. Treatment may be medical or surgical, and intrauterine procedures can contribute to adhesion risk. Hooker and colleagues' review of 912 women found intrauterine adhesions in 19.1% after miscarriage, with repeated curettage an important risk factor.[13] This pooled rate is not the risk caused by one procedure in every patient.
• Psychological distress. A positive test, subsequent heartbeat and later pregnancy loss can produce a substantial emotional burden.
• Time. hCG must fall and menstruation and the endometrium recover; the interval varies and can be months. In Demko and colleagues' retrospective dataset, the estimated probability of obtaining at least one euploid embryo in a tested cycle was approximately 85% at age 35, 75% at 40 and 45% at 44.[11] These are population estimates influenced by embryo numbers and selection, rather than a personal forecast.

Many centres decline transfer of certain aneuploid results. Where a transfer is considered, detailed counselling and a prenatal-testing plan may be required. A signed consent form alone does not determine a centre's policy.

05 Mosaic, segmental and inconclusive results

The preceding concerns about clear nonmosaic whole-chromosome findings should not be applied indiscriminately to other categories.

Mosaic results

ESHRE's 2022 recommendations use a 50% boundary for lower- versus higher-range findings. Low-range mosaic embryos may be considered for transfer and should not automatically be discarded; the document could not make a uniform recommendation for higher-range embryos.[8] PGDIS also discusses poorer outcomes with higher mosaic levels and relatively favourable outcomes for segmental mosaic findings, and advises against treating every mosaic result as unusable.[9] ASRM's 2023 review reported confirmation of the same abnormality in the fetus or newborn in fewer than 1% of reported mosaic transfers with relevant outcome information.[10] Incomplete follow-up and differences in testing limit interpretation; this is not a personal guarantee.

Segmental findings

A genetic counsellor reviews the size and position of the deletion or duplication, involvement of known disease-associated regions, mosaic status, other abnormalities and laboratory methods. These details guide individualized discussion rather than a blanket transfer rule.

Inconclusive results

An inconclusive test is not an aneuploid diagnosis. In Neal and colleagues' study, 2.5% of biopsy samples had no result. Rebiopsy produced a diagnosis in 95.2%, with 56.0% euploid findings.[14] Whether to warm and rebiopsy depends on embryo condition, laboratory experience, the value of additional information and the risks of repeat manipulation. Rebiopsy to resolve a failed test differs from routinely rebiopsying a mosaic result to “confirm” it; ESHRE does not recommend the latter.[8]

06 Reviewing the only remaining blastocyst

A report describing the only remaining embryo as unsuitable can be distressing. The following four steps organize the discussion rather than prescribe a universal decision.

1. Identify the exact finding

Review whether it is whole-chromosome or segmental, mosaic or nonmosaic, and a loss or gain. If necessary, ask the laboratory to explain estimated copy number and plots, particularly when values are close to classification thresholds. A clear whole-chromosome loss and a borderline signal raise different questions.

2. Allow time before an irreversible storage decision

If clinic policy and finances allow, continued storage can leave time for counselling and emotional adjustment. Storage preserves an option for later review; it does not establish that transfer will become appropriate or safe.

3. Seek genetic counselling for mosaic, segmental or inconclusive findings

Bring the full report and laboratory explanation. Review possible transfer outcomes, further testing, prenatal diagnosis and the centre's policy. Avoid making a transfer or disposal decision solely from a broad label.

4. Review the previous cycle and future options

Discuss oocyte maturity, fertilization method, blastocyst development, sperm quality and whether another stimulation strategy or another reproductive option is appropriate. Recovery after an unsuccessful transfer may use time relevant to ovarian ageing. PGT-A does not create additional embryos, and ASRM does not recommend its routine use for all IVF patients.[2] Its potential benefits and limitations should be considered for the particular patient.

07 Planning beyond the report

The clinical purpose of PGT-A is to supply chromosome information that can guide embryo selection. Its value also depends on how that information affects miscarriage risk, time, costs, embryo availability and patient preferences. Testing does not guarantee a healthy child, and its routine benefit for everyone has not been demonstrated.[2]

Read the exact classification before deciding what “abnormal” means. Clear nonmosaic whole-chromosome results, mosaic findings, segmental changes and failed tests need distinct explanations. A discussion with the treating team and genetic counsellor can then address storage, transfer policy, prenatal diagnosis and future treatment without promising a particular outcome.

Appendix: common report terms

Detection of uniparental disomy, polyploidy, balanced translocations and inversions depends on the assay; routine copy-number PGT-A does not reliably exclude all of these. Some SNP-based methods can identify certain ploidy abnormalities. Check the laboratory's stated scope. The sample is trophectoderm rather than the inner cell mass.

References and evidence table

The manuscript's search ended on 17 October 2026. This is a narrative review rather than a registered systematic review. For web publication on 24 October, study-specific endpoints, diagnostic uncertainty and assay-dependent limitations were clarified. Numbers from different designs should be interpreted with their denominators and populations.

Medical information and image disclosure

This educational article does not replace examination or genetic counselling. The scenario summarizes common consultation questions and does not identify a particular patient. Transfer eligibility is determined by the treating team, genetic counsellor, full report, laboratory explanation and centre policy.

Quantitative results are attributed in the evidence table. The section 06 decision diagram is an editorial synthesis, not a society recommendation. Five original infographics are retained in Chinese and localized for English and Russian. The new cover and seven supplied artistic illustrations were made with AI assistance; they are conceptual images, not actual embryo images, diagnostic findings or predictions of pregnancy.