PGT-A (preimplantation genetic testing for aneuploidy) screens IVF embryos for abnormal chromosome counts across all 23 pairs of human chromosomes, including the 22 numbered pairs and the sex chromosomes X and Y. The goal is to identify embryos with the right number of chromosomes before transfer, improving the chances of a successful pregnancy and reducing the risk of miscarriage.
What PGT-A Actually Detects
The core purpose of PGT-A is detecting aneuploidy, which means an embryo has too many or too few chromosomes. A normal human embryo has 46 chromosomes arranged in 23 pairs. When something goes wrong during cell division, an embryo can end up with an extra copy of a chromosome (trisomy) or a missing one (monosomy). PGT-A catches both.
Beyond simple extra or missing chromosomes, the test can also flag several other types of errors: segmental abnormalities (where only part of a chromosome is duplicated or deleted), uniparental disomy (where both copies of a chromosome come from the same parent instead of one from each), and polyploidy (where the embryo has entire extra sets of chromosomes). These are all structural problems at the chromosome level that would likely prevent a healthy pregnancy.
The chromosomal conditions most people associate with aneuploidy include Down syndrome (an extra chromosome 21), Edwards syndrome (extra chromosome 18), and Patau syndrome (extra chromosome 13). But PGT-A isn’t limited to those. It screens every chromosome, so it picks up abnormalities that would cause an embryo to fail to implant or miscarry early, often involving chromosomes that don’t produce recognizable syndromes because the pregnancy simply can’t continue.
What PGT-A Does Not Test For
PGT-A is strictly a chromosome-counting tool. It does not detect single-gene disorders like cystic fibrosis, sickle cell disease, or Huntington’s disease. That requires a different test called PGT-M (preimplantation genetic testing for monogenic disorders), which targets specific known mutations in a family.
There’s also PGT-SR, designed for families where one partner carries a structural chromosome rearrangement like a balanced translocation or inversion. While PGT-A may pick up the unbalanced outcomes of such rearrangements, it can’t detect balanced ones where no genetic material is actually gained or lost. Depending on the testing platform used, PGT-A also has limits on how small a chromosomal change it can reliably detect, with some methods missing abnormalities smaller than 5 to 10 million base pairs.
How the Test Is Done
PGT-A requires IVF. After fertilization, embryos are grown in the lab for five days until they reach the blastocyst stage, at which point they contain roughly 80 to 100 cells. An embryologist removes four to six cells from the trophectoderm, the outer layer of the blastocyst that will eventually become the placenta. The inner cell mass, which develops into the baby, is left untouched.
Those biopsied cells are sent to a genetics lab for analysis while the embryos are frozen. Results typically classify each embryo into one of three categories: euploid (normal chromosome count), aneuploid (abnormal), or mosaic (a mix of normal and abnormal cells).
Understanding Mosaic Results
Mosaic embryos are one of the more confusing parts of PGT-A results. In testing terms, mosaicism is defined as an embryo showing between 20% and 80% abnormal DNA content in the biopsied cells. Below 20%, the embryo is classified as euploid. Above 80%, it’s classified as aneuploid.
Mosaic embryos occupy a gray zone. They have lower success rates than fully euploid embryos, but some do result in healthy pregnancies. Medical guidelines from multiple professional societies agree on a few points: euploid embryos should be transferred first, embryos with lower levels of mosaicism tend to have better outcomes than those with higher levels, and mosaic embryos with monosomies generally fare better than those with trisomies. Certain chromosomes are flagged as lower priority for mosaic transfers, particularly chromosomes 13, 14, 15, 16, 18, and 21, because of their association with recognized syndromes, growth restriction, or other complications. If you do transfer a mosaic embryo, prenatal testing through amniocentesis is recommended.
How Accurate Is PGT-A
A large meta-analysis found that PGT-A is highly reliable for its two main calls. When the test labels an embryo aneuploid, that result is confirmed about 89% of the time. When it labels an embryo euploid, the result holds roughly 94% of the time, and the actual misdiagnosis rate after transferring a euploid embryo is less than 1%.
Mosaic results are less reliable. When PGT-A flags an embryo as mosaic, only about 53% are confirmed mosaic or aneuploid on further testing. The other half turn out to be euploid. This means some healthy embryos may be deprioritized based on a mosaic label, which is one reason clinics don’t automatically discard them. About 22% of mosaic embryo transfers result in a confirmed euploid pregnancy.
It’s worth remembering that PGT-A is a screening test, not a diagnostic one. The biopsy samples only the outer cell layer, which doesn’t always perfectly represent the genetics of the entire embryo. That’s why confirmatory prenatal testing is often recommended, especially after mosaic transfers.
Who Benefits Most From PGT-A
The rate of chromosomally abnormal embryos rises sharply with age. For women under 35, roughly 25% of eggs are aneuploid. By age 40 and older, that number climbs above 75%. This is why PGT-A tends to have the greatest impact for women over 35, where it significantly improves both clinical pregnancy rates and live birth rates per transfer.
For younger women, the benefit is more nuanced. Live birth rates per transfer are still higher with PGT-A, but because younger women produce more euploid embryos to begin with, the test adds fewer screening gains relative to the cost. One analysis found PGT-A isn’t cost-effective for women under 36. That said, even younger patients with specific risk factors can benefit substantially.
PGT-A shows particularly strong results for people with a history of recurrent miscarriage. In one study of recurrent miscarriage patients, early pregnancy loss dropped from 75% without PGT-A to 18% with it. For patients with repeated implantation failure (where transferred embryos simply don’t take), PGT-A more than doubled the live birth rate per transfer, from 19% to nearly 48%. These are the populations where screening out chromosomally abnormal embryos makes the most dramatic difference.
Impact on IVF Outcomes
Transferring a PGT-A tested euploid embryo results in a live birth rate of about 48% per transfer, compared to roughly 35% per transfer without testing. That improvement comes from two places: fewer failed implantations and fewer miscarriages, since the most common cause of both is chromosomal abnormality in the embryo.
The tradeoff is that PGT-A reduces the number of embryos available for transfer, since aneuploid embryos are set aside. For someone who produces many embryos, this is rarely a problem. For someone with very few embryos, especially an older patient, the test might leave them with no euploid embryos to transfer at all. That result is still informative (it likely explains why previous cycles failed), but it can be difficult to receive. The decision to use PGT-A often comes down to whether you’d rather transfer fewer embryos with higher individual success rates, or transfer more embryos knowing some will be chromosomally abnormal.