IVF · Genetic testing
Pre-implantation genetic testing — what PGT-A, PGT-M, and PGT-SR actually tell us
Pre-implantation genetic testing (PGT) is the analysis of embryos for genetic information before transfer. There are three forms: PGT-A (aneuploidy screening), PGT-M (monogenic disorders), and PGT-SR (structural rearrangements). Each has clear indications and clear limits. This page describes when each is appropriate, what the test does and does not tell us, and how to integrate PGT into an IVF cycle.
The three types
- PGT-A (aneuploidy) — screens embryos for the correct number of chromosomes (46, XX or 46, XY). The most common form.
- PGT-M (monogenic) — tests for a specific single-gene disorder when one or both partners are known carriers (e.g. cystic fibrosis, thalassaemia, Huntington disease).
- PGT-SR (structural rearrangements) — tests for chromosomal rearrangements (translocations, inversions) where one parent is a known carrier.
Indications for PGT-A
The honest picture is more nuanced than commonly presented. Reasonable indications include:
- Recurrent pregnancy loss with confirmed aneuploidy in prior losses
- Recurrent IVF implantation failure with morphologically good embryos
- Advanced maternal age (often ≥38) — selectively, with explicit counselling about the value and limits
- Patient preference after structured counselling about benefits, costs, and uncertainty around mosaic results
PGT-A is not a universal recommendation. In good-prognosis patients under 35 with normal embryo morphology, PGT-A may not improve live birth rates and adds cost and the small biopsy risk.
Indications for PGT-M and PGT-SR
- Known monogenic disease in one or both partners — reasonable to discuss PGT-M to avoid transmission
- Known balanced translocation or inversion in one partner — PGT-SR substantially improves outcomes by selecting balanced embryos
- Family history of severe genetic disease — structured genetic counselling first
How PGT is performed
- Embryos are cultured to blastocyst stage (day 5 or 6)
- A small number of trophectoderm cells (5–10) are biopsied from each blastocyst
- Embryos are vitrified and stored
- The biopsied cells are analysed by next-generation sequencing (NGS)
- Results are returned within 1–3 weeks
- Selected embryos are warmed and transferred in a subsequent cycle
Mosaic embryos — the grey area
Modern NGS can detect mosaicism — embryos where some cells are euploid and others are aneuploid. The clinical management of mosaic embryos is one of the most debated areas in PGT. Low-level mosaic embryos transferred in selected cases have resulted in healthy live births. The PGDIS framework guides the discussion. Patients are counselled in advance about what a mosaic result means and what choices it would create.
What PGT does not do
- It does not test for every genetic condition — PGT-A tests chromosome number, not single-gene mutations
- It does not guarantee a healthy baby — pregnancy and obstetric risks remain
- It does not replace prenatal screening (NIPT, combined screening, anomaly scan) during pregnancy
- It does not always give a definitive result — some embryos return inconclusive or mosaic results
- It does not eliminate the chance of miscarriage entirely
Honest counselling
PGT is presented as one option, with explicit cost, the realistic probability that it will help, and the biopsy risk. The conversation includes the option to not do PGT-A — particularly in good-prognosis patients where the evidence of benefit is weakest.
When PGT should be discussed
- Recurrent pregnancy loss (≥2 losses)
- Repeated IVF implantation failure
- Advanced maternal age in patients planning IVF
- Known genetic disease in either partner
- Family history of chromosomal abnormality
- Patient request after counselling
Guidelines we follow on this topic
- ESHRE PGT guidelines
- PGDIS position statements
- ASRM committee opinion on PGT
- NICE fertility guidance
Related reading
PGT-A should be offered with appropriate counselling about its role, limitations, and the possibility of inconclusive (mosaic) results. It does not improve cumulative live birth rate in unselected populations.
CONTINUE READING
Types of PGT
| Type | What it detects |
|---|---|
| PGT-A | Chromosome number (aneuploidy) |
| PGT-M | Single-gene conditions |
| PGT-SR | Structural rearrangements |
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Pre-implantation Genetic Testing is one part of the broader IVF and fertility programme. The main IVF pillar covers individualised protocols, success counselling, and long-term reproductive planning.
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