Preimplantation genetic testing (PGT): when is it needed?

Preimplantation genetic testing (PGT) is an umbrella term that covers four distinct tests performed on embryos before transfer. Of these, the most widely used is PGT-A, which examines whether embryos carry chromosomal abnormalities. It is also the one that has given rise to the most misconceptions. It is often judged by the wrong question: “Does it increase the cumulative live birth rate?”. The right question is different: “Does it help us make better clinical decisions?” — because PGT-A does increase the live birth rate per transfer, reduces miscarriages and shortens the path to a baby. When the question is framed this way, the answer from the recent evidence becomes clear.

PGT-A is a diagnostic tool, not a treatment. Just as a mammogram does not treat cancer but guides the right treatment, PGT-A does not make an embryo more capable of producing a pregnancy — it tells us which embryo is worth transferring.

Four types of PGT — what each one does

“PGT” refers to four distinct tests:

PGT-A (Aneuploidy) — checks whether embryos have the normal number of chromosomes.
PGT-M (Monogenic) — looks for a specific inherited gene disorder when the parents are known carriers (e.g. β-thalassaemia, cystic fibrosis, Huntington’s disease).
PGT-SR (Structural Rearrangement) — detects structural chromosomal abnormalities in couples with a known translocation or inversion.
PGT-HLA (HLA matching) — selects embryos that are HLA-compatible with an existing sick sibling needing a stem-cell transplant (e.g. for thalassaemia, Fanconi anaemia). Umbilical cord blood from the resulting newborn can be used to treat the affected sibling.

PGT-M, PGT-SR and PGT-HLA have clear, narrowly defined indications: a known genetic disease, a chromosomal translocation, or the need for HLA compatibility with a sick sibling. PGT-A applies to a much broader group of infertile couples and is the focus of what follows.

Why PGT-A exists: the biological reality

Aneuploidy (an abnormal number of chromosomes) is the primary cause of implantation failure and early miscarriage. More than half of first-trimester miscarriages are due to chromosomal abnormalities of the embryo.

How common is aneuploidy? In a study of 15,169 trophectoderm biopsies, the rate of aneuploidy in blastocysts rises steadily with the woman’s age:

Ages 26-30: 23% aneuploid blastocysts
Ages 35-37: 37.5%
Ages ≥42: 89.6%

The crucial point: morphological grading of embryos by the embryologist cannot detect these abnormalities. Even top-grade blastocysts (≥3AA) are aneuploid in 44% of cases. Without genetic testing, we are essentially transferring blind — and many failures are due to exactly that.

How modern PGT-A is performed

Trophectoderm biopsy on a day-5 or day-6 blastocyst: 5-10 cells are taken from the trophectoderm. The inner cell mass, which will become the baby, remains untouched.
Next-Generation Sequencing (NGS) of all 23 chromosome pairs.
Classification: euploid → transfer, aneuploid → no transfer, mosaic → individualised discussion.
Transfer is performed in a subsequent cycle after freezing — giving time to study the results.

What the recent evidence shows (2021-2025)

When PGT-A is used properly, the evidence is clear:

Substantial reduction in miscarriages: across three recent randomised trials, miscarriages fall from 12.6-30.1% to 4.5-13.9%.
Twofold live birth rate per transfer: 41.5% with PGT-A vs 16.2% without (2.6×).
57 days faster to a live birth or to the decision to stop trying: 74 vs 131 days on average.
Fewer transfers per patient: 1.1 on average with PGT-A vs 1.3 without (15% fewer).
Better neonatal outcomes in singleton births (meta-analysis of 42 studies, 260,665 participants).

Important clarification: the cumulative live birth rate is not higher with PGT-A — because the euploid embryos would eventually have produced a pregnancy anyway. This does not mean PGT-A is useless. It means it serves a different, equally valuable role: fewer miscarriages, fewer pointless transfers, and a faster path to the baby.

How reliable is it?

Biopsy concordance with the embryo: 94% for euploid calls (meta-analysis of 26 studies, 1,271 blastocysts).
Whole-chromosome aneuploid calls (an entire chromosome missing or extra): highly reliable (PPV 97-100%; in one non-selection study, no live births from 102 such transfers). Segmental aneuploid calls (partial chromosomal abnormalities) are less reliable.
Mosaic calls (5-20% of results): less reliable — these require genetic counselling and case-by-case decisions.
“No result” (2-6%): uncommon; a repeat biopsy is possible.

When PGT-A is offered

PGT-A is considered in:

Advanced maternal age (particularly ≥35 years).
Recurrent pregnancy loss.
Repeated implantation failure after transfer of good-quality embryos.
Multiple blastocysts where selection for single embryo transfer is required.
Couples who want a clear answer before deciding whether to continue trying.

Note: PGT-A used to be considered only for women of advanced reproductive age. Newer data show that even at peak fertility (26-30 years), about 23% of blastocysts are aneuploid — so PGT-A has value across all ages.

In Greece, PGT is subject to genetic counselling and approval by the National Authority for Medically Assisted Reproduction (ΕΑΙΥΑ). For PGT-A on the grounds of advanced reproductive age, the current directive (ΕΑΙΥΑ, 11 Aug 2022) requires a medical note from the treating physician clarifying:

whether the patient’s own oocytes will be used, or donor oocytes;
if donor oocytes are used: the donor’s age, whether one or more donors are involved, and whether the licence applies to cryopreserved embryos from that donor or to a fresh cycle.

Under the most recent update (April 2026), the genetic-counselling report supporting a PGT licence may also be signed by a professional with the specialty of Laboratory Genetics.

Summary: questions for your doctor

Which type of PGT are we talking about — PGT-A, PGT-M or PGT-SR?
What is the specific indication in our case?
How many of the embryos are expected to reach the blastocyst stage for testing?
What happens if the results show mosaicism?
How will PGT-A change the transfer decision — and what benefit do we expect?

PGT-A is a diagnostic tool, not a treatment. Just as a mammogram does not treat cancer but guides the right treatment, PGT-A does not make an embryo more capable of implanting — it tells us which embryo is worth transferring. When the question is framed this way, its value is clear: fewer miscarriages, fewer pointless transfers, a faster path to the baby.