Preimplantation genetic testing sits at one of the more technically complex intersections in reproductive medicine, and the terminology around it creates confusion that is entirely understandable. Three different types of testing share the same abbreviation, each designed to answer a different clinical question, and the recommendation for which to use depends on factors that are specific to each couple’s situation rather than a general preference for one approach over another. Getting clear on what each type does and when it applies makes the conversation with a specialist considerably more productive than going in without that foundation.
The common thread across all three is that the testing is performed on embryos before they are transferred during an IVF cycle, which is what makes it preimplantation. Beyond that shared characteristic, the similarities between the three types are limited, and the differences between them are meaningful enough to affect which is relevant for a given clinical situation.
PGT-A: Testing for Chromosomal Number Errors
PGT-A stands for preimplantation genetic testing for aneuploidies, where aneuploidy refers to an abnormal number of chromosomes in an embryo. A typical human embryo should have 46 chromosomes arranged in 23 pairs. Embryos with an incorrect number, whether more or fewer than expected, are significantly less likely to implant successfully and more likely to result in miscarriage if they do implant. Some chromosomal abnormalities that result in live birth, such as trisomy 21 which causes Down syndrome, are also detected through this approach.
PGT-A is most commonly recommended for women of advanced maternal age, for couples who have experienced recurrent pregnancy loss, and for those who have had multiple failed IVF transfer cycles. The rationale is that chromosomal errors in embryos become more common with age, and selecting embryos with a normal chromosomal count for transfer improves the likelihood of a successful outcome. It is worth understanding that PGT-A does not improve the overall number of normal embryos a cycle produces. It identifies which among the available embryos are more likely to lead to a successful pregnancy.
PGT-M: Testing for Specific Inherited Conditions
PGT-M, which stands for preimplantation genetic testing for monogenic disorders, takes a different approach. Rather than looking at chromosome number across the full genome, it is designed to detect whether an embryo has inherited a specific genetic variant that causes a known inherited condition. This type of testing is relevant when one or both partners carry a gene variant associated with a condition such as cystic fibrosis, sickle cell disease, Huntington’s disease, or any of the hundreds of other single-gene conditions for which the causative variant has been identified.
The testing is highly specific and requires preparation before the IVF cycle begins, because the laboratory needs to design a custom test based on the specific genetic variant in the family. This preparation period, typically several weeks, is worth accounting for in the timeline when planning a cycle. The goal is to identify embryos that have not inherited the causative variant, allowing those embryos to be prioritized for transfer.
Medgenome offers PGT testing as part of its reproductive health diagnostics portfolio, with the laboratory infrastructure and genetic expertise needed to support both PGT-A and PGT-M workflows. The clinical detail of how pgt testing is conducted and what the process involves from sample collection through to results is available through the diagnostic page, which covers the technical approach and the clinical applications across different patient groups.
PGT-SR: Testing for Structural Chromosomal Rearrangements
PGT-SR is the least commonly discussed of the three types and applies to a specific situation: when one partner carries a chromosomal rearrangement rather than a single-gene variant or a numerical chromosomal error. Structural rearrangements include translocations, where segments of chromosomes have swapped positions, and inversions, where a chromosomal segment has been reversed. Carriers of these rearrangements may be clinically unaffected themselves but are at higher risk of producing embryos with unbalanced chromosomal arrangements, which can result in miscarriage or significant developmental conditions.
PGT-SR identifies which embryos have inherited the chromosomal rearrangement in a balanced or normal form, and which have an unbalanced arrangement, allowing transfer to be directed toward those with a better prognosis. For couples where a chromosomal rearrangement has been identified through karyotyping, often following recurrent miscarriage or a previous affected pregnancy, this type of testing provides a pathway to IVF that significantly improves the likelihood of a successful outcome compared to unscreened transfer.
The decision about which type of testing applies, or whether a combination is appropriate, is one that belongs in a conversation with a genetic counselor and a reproductive specialist who can assess the specific clinical picture. The broader context of preimplantation genetic testing and how the different types connect to specific clinical indications is covered through the same Medgenome diagnostic resource, which gives a structured overview before a specialist consultation takes place.
