PGT explained: PGT-A, PGT-M and PGT-SR

What PGT actually does

Preimplantation genetic testing takes a small biopsy of cells from an embryo, usually at blastocyst stage, and sends them for genetic analysis before a decision is made about which embryo to transfer. The embryo is frozen while the result is awaited.

There are three distinct tests and they answer different questions. It helps to be clear which one your clinic is actually offering, because they are sometimes bundled together in conversation.

PGT-A

PGT-A screens for aneuploidy, meaning an embryo with the wrong number of chromosomes. Aneuploid embryos are a common reason implantation fails or a pregnancy miscarries, and the chance of an embryo being aneuploid rises with the age of the eggs used.

PGT-M

PGT-M tests for a single named genetic condition that a specific family already carries, such as a condition identified through previous genetic counselling. It is built around that known mutation, so it cannot be used as a general screen.

PGT-SR

PGT-SR looks for structural rearrangements of chromosomes, such as a translocation carried by one parent, which can lead to embryos with unbalanced chromosome material.

The HFEA rates several embryo add-ons, including PGT-A for people without a known indication, against the evidence available for improving the chance of a baby, not just the chance of a positive test result. Ask your clinic to talk you through the current rating and what it is based on.

Who it applies to

• PGT-M and PGT-SR are usually offered where a specific genetic condition or chromosome rearrangement has already been identified in one or both partners.
• PGT-A is sometimes offered more broadly, including to people with no known genetic condition, as a way of trying to choose between embryos.
• Your clinic's genetics team, not just the embryology team, should be involved in decisions about PGT-M and PGT-SR.

What it costs you

PGT is an added cost on top of a cycle, and it also adds time, because embryos are frozen while waiting for results rather than transferred fresh. Some embryos may not be suitable for biopsy, and testing does not guarantee a usable result for every embryo tested.

What the evidence does and does not show

PGT-A can reduce the chance of transferring an embryo that will not implant or will miscarry due to a chromosome problem. It has not been shown to increase the overall chance of taking home a baby for everyone who has it, and some embryos flagged as abnormal are mosaic rather than clearly abnormal, which complicates decisions.

Questions worth asking your clinic

• What specifically will this test tell me, and what will it not tell me?
• How many embryos are likely to be suitable for biopsy in my case?
• What happens if a result comes back mosaic rather than clearly normal or abnormal?
• What is the added cost and how much extra time will it add to my treatment?
• What does the current HFEA add-on rating say about this in cases like mine?

Mosaic results

A mosaic result means the embryo has a mix of normal and abnormal cells in the sample tested. This does not necessarily mean the embryo cannot lead to a healthy pregnancy, and decisions about whether to transfer a mosaic embryo are individual and should involve genetic counselling.