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PGT-A (Preimplantation Genetic Testing for Aneuploidy): What It Tests, Mosaic Results, and What the Evidence Shows
IVF Guide·

PGT-A (Preimplantation Genetic Testing for Aneuploidy): What It Tests, Mosaic Results, and What the Evidence Shows

PGT-A screens embryos for chromosome number before transfer. This guide explains what the test checks, how to read euploid, aneuploid and mosaic results, and where the evidence is — and is not — strong.

Written by Sun SiyingMedically reviewed by Daniel Sun (Endocrinology)Reviewed

What PGT-A is and what it tests

Preimplantation genetic testing for aneuploidy (PGT-A) is a laboratory procedure used during in vitro fertilization (IVF) to check whether an embryo has the expected number of chromosomes before it is transferred to the uterus. Aneuploidy means a cell has too many or too few chromosomes — for example, three copies of chromosome 21 instead of the usual two. Because chromosome imbalances are a leading cause of embryos not implanting or ending in miscarriage, many clinics offer PGT-A as a way to help prioritize which embryo to transfer.

It helps to separate PGT-A from its relatives. PGT-M looks for specific single-gene conditions that run in a family, and PGT-SR is used when one parent carries a chromosomal rearrangement such as a balanced translocation. PGT-A, by contrast, screens the chromosome count across all 23 pairs. It is a screening test, not a diagnosis of every possible genetic condition, and it does not replace counseling with a genetics professional.

How the biopsy and testing work

PGT-A is performed on a blastocyst — an embryo that has reached about day 5 or 6 of development. At this stage a small number of cells (typically 5–10) are taken from the trophectoderm, the outer layer that becomes the placenta, using a fine laser-assisted biopsy. The inner cell mass, which becomes the fetus, is left untouched. The biopsied cells are sent for genetic analysis while the embryo itself is frozen and stored.

Embryo biopsy under laser-assisted micromanipulation during PGT-A

The genetic material is then amplified and examined. Most laboratories today use next-generation sequencing (NGS) to count chromosomes. Because the embryo is frozen (vitrified) during testing, the timeline usually shifts: the transfer, if a suitable embryo is chosen, generally happens in a later cycle rather than the same one. This freeze-all approach is standard at many clinics and keeps the testing step separate from the transfer step. The result comes back as a classification rather than a single yes-or-no, which is why understanding the report matters.

Understanding the report: euploid, aneuploid, mosaic

A report will usually place an embryo in one of three groups. Euploid means the tested cells showed the normal number of chromosomes — 46 in total. Aneuploid means a whole extra or missing chromosome was found, such as trisomy or monosomy. Mosaic means the sample contained a mixture: some cells looked euploid and some aneuploid.

Mosaic results are reported with an estimated percentage. A low mosaic result might show, for example, 20–40% aneuploid cells in the sample, while a high mosaic result shows a larger proportion. Important: the biopsy samples only a handful of cells from the outer layer, so the percentage is an estimate of that sample, not a precise measurement of the whole embryo.

What mosaic embryos mean

Mosaic embryos used to be discarded routinely. Practice has shifted as clinics observed healthy births from embryos reported as mosaic. A mosaic result does not mean an embryo cannot lead to a live birth; it means the tested sample was inconsistent. Because the placenta (trophectoderm) and the fetus (inner cell mass) can differ, and because the sample is tiny, the true makeup of the embryo may be better than the report suggests — though it can also be worse.

Clinics differ in how they use mosaic embryos. Many prioritize euploid embryos first, then consider mosaic embryos when no euploid is available, and may counsel patients on the trade-offs. There is no single rule, and the decision is usually made together with the medical team based on the specific results and the patient's history.

What the evidence does and does not show

PGT-A can lower the chance of transferring an embryo with a major chromosome abnormality, and for some patients — particularly those of advanced maternal age or with repeated pregnancy loss — it is associated with higher live-birth rates per transfer and fewer miscarriages. That is a real and useful benefit.

What the test does not do is promise a pregnancy. It does not increase the total number of healthy babies a person can have across all their embryos, because it is a selection tool, not a treatment that improves embryo quality. Biopsy carries a small risk, and testing is not perfectly accurate: mosaicism, lab differences, and the limits of sampling mean a result should be interpreted, not treated as absolute. PGT-A also does not screen for most single-gene diseases or for many traits and conditions.

Large registry datasets, such as those collected by SART and reported by CDC, show that maternal age strongly affects how many embryos are euploid; the share drops noticeably after about age 35 and more steeply after 40. PGT-A is one way clinics account for that biology, but it does not change the underlying number of healthy embryos a person produces.

Questions to ask your clinic

  • Which embryos will you biopsy, and on what day of development?
  • How do you report and use mosaic results?
  • What are the accuracy limits of your testing laboratory?
  • If no euploid embryo is available, what is your policy on mosaic transfer?
  • How do the costs compare with transferring without testing?

Talking these through before a cycle helps you understand what the result will and will not tell you, and avoids surprises later.

References

Related reading on this site: blastocyst grading, frozen embryo transfer, IVF success rates, choosing a clinic.

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