PGT-A (embryo genetic testing): what it can and can't tell you

PGT-A — preimplantation genetic testing for aneuploidy — checks whether an embryo has the right number of chromosomes before it is transferred. It is one of the most useful and one of the most oversold things in IVF. Here is what it involves and how to think about it.

What is done

1. Biopsy. On day 5 or 6, when the embryo is a blastocyst, the embryologist makes a small opening in the shell with a laser and removes 5–8 cells from the outer layer (the trophectoderm — the future placenta), not from the inner cell mass. It's precise, delicate work.
2. Freezing. The embryo is vitrified straight after biopsy and waits.
3. Testing. The cells go to a genetics lab which counts the chromosomes (usually by next-generation sequencing). Results take one to three weeks.
4. Transfer in a later frozen cycle of an embryo reported as normal.

The result words

• Euploid — normal chromosome number. Recommended for transfer.
• Aneuploid — a whole chromosome missing or extra. Not transferred; almost never leads to a healthy pregnancy.
• Mosaic — the sample contains both normal and abnormal cells. Depending on the level and the chromosome involved, some mosaic embryos are transferred (often after euploid ones) and many have produced healthy children. This category is where genetic counselling matters.
• No result / inconclusive — a few percent; the embryo can sometimes be re-biopsied.

Who benefits most

• Patients over about 37–38, where a large share of embryos are aneuploid: testing avoids transferring embryos that will fail or miscarry, saving months and heartbreak.
• Recurrent miscarriage or repeated implantation failure.
• Couples who want to transfer one embryo at a time with more confidence.

The limits, plainly

• It tests the placenta cells, not the baby cells. Usually they agree; sometimes they don't — which is why mosaic results exist and why a "normal" result is very reassuring but not a guarantee.
• It counts chromosomes; it doesn't check genes. PGT-A does not detect single-gene diseases (that's PGT-M) or guarantee a healthy child.
• A euploid embryo can still fail to implant. Around 60–70% implant, not 100%.
• It can't make an embryo better. If you have few embryos, testing may leave you with none to transfer — a genuine risk to weigh with your doctor when numbers are low.
• It costs, and for younger patients with several good embryos the extra chance of a baby per cycle is small; the main gain is fewer failed transfers on the way.

What good labs do

Biopsy is a skill; results depend on the embryologist as much as the genetics lab. Reasonable questions: how many biopsies does the lab do a year, what is its survival rate after biopsy and warming, and what proportion of results are inconclusive? Clear answers are a good sign.