PGT-A vs PGD vs PGS: Genetic Testing of Embryos in IVF

Which Test Are We Talking About?

When couples plan IVF they often hear a string of abbreviations: PGS, PGD, PGT-A, PGT-M. It is easy to be confused, and the terms have changed. Here I explain what each test does, who may benefit, what its limits are, and what Indian law says.

This article is for education only. Whether genetic testing is right for you depends on your individual circumstances and should be decided after counselling with your fertility specialist. Any statistic below is approximate, tied to an age group and cannot predict your own outcome.

What Is PGT-A?

Humans normally have 46 chromosomes. An embryo with the right number is called euploid; one with extra or missing chromosomes is aneuploid. Aneuploidy is a leading cause of failed implantation and early miscarriage, and some aneuploidies lead to conditions such as Down syndrome (an extra chromosome 21).

PGT-A (preimplantation genetic testing for aneuploidy) checks the number of chromosomes in an IVF embryo before transfer, so that a chromosomally normal embryo can be prioritised.

PGS, PGD, PGT-A, PGT-M, PGT-SR: The Terminology

The terms were updated in 2017 to make them clearer:

PGT-A looks at chromosome number in general, and typically applies to couples with no known family genetic disease. PGT-M and PGT-SR are targeted tests for couples who know they carry a particular condition or rearrangement.

How Embryo Biopsy Works

Testing needs a small sample of cells from the embryo.

1. Culture to blastocyst. Embryos are grown for five or six days until they reach the blastocyst stage, when they contain around a hundred cells with two parts: the inner cell mass (which becomes the baby) and the trophectoderm (which becomes the placenta).
2. Biopsy. Using a laser and a fine pipette, an embryologist takes a few cells from the trophectoderm. The inner cell mass is not touched.
3. Freezing. Embryos are usually frozen by vitrification, a fast-freezing technique, while the results are awaited. See embryo freezing and vitrification.
4. Analysis. The cells are sent for genetic analysis, commonly next-generation sequencing, which takes some days.
5. Frozen transfer. A suitable embryo is thawed and transferred in a later cycle. Read the frozen embryo transfer guide and see my frozen embryo transfer service.

The skill of the embryology team matters at every step. I work closely with the embryologists on embryo culture and selection, and biopsy and freezing are laboratory skills where experience counts. Other tools, such as embryo grading and time-lapse monitoring, help assess how an embryo looks and develops, but they cannot show its chromosomes.

Who May Benefit From PGT-A?

PGT-A is not needed for everyone. It is most often discussed in these situations.

Advanced maternal age

Chromosomal errors in eggs become more common with age, so the proportion of aneuploid embryos rises. Published studies of PGT-A cycles report roughly the following ranges. They are approximate and vary between laboratories and populations:

• Under 35: about a quarter to a third or so of tested blastocysts
• 35 to 37: around 40 to 50 per cent
• 38 to 40: roughly 55 to 65 per cent
• Over 40: often well over two-thirds

Read more in age-related fertility decline, the IVF age limit in India and IVF success rate by age. Where the chance of getting a normal embryo from a woman's own eggs is very low, donor egg IVF may also be discussed.

Recurrent miscarriage

After two or more losses, an aneuploid embryo is a frequent cause. PGT-A may be considered alongside other tests. See recurrent miscarriage causes and treatment and what happens after a blighted ovum.

Repeated implantation failure

When several good-looking embryos have not implanted, chromosomal errors may be one explanation, but uterine factors should be checked too. See implantation signs, window and failure, chronic endometritis and IVF failure and IVF failure reasons and next steps.

Known genetic conditions

PGT-M or PGT-SR is used when the couple carries a specific gene mutation or chromosome rearrangement.

What about embryo arrest?

Many embryos that stop developing in the laboratory carry severe chromosomal errors. See embryo arrest causes. PGT-A can only test embryos that reach the blastocyst stage, so if none does, there is nothing to test.

Limitations You Should Know

• Mosaicism. Some embryos contain a mix of normal and abnormal cells. The biopsy takes only a few cells from the trophectoderm, which may not represent the whole embryo. Some mosaic embryos can result in healthy babies, and their use is decided case by case after counselling.
• It is a screening test, not a guarantee. A chromosomally normal embryo can still fail to implant because of uterine, hormonal, immune or other factors. Pregnancy screening and diagnostic tests are still recommended.
• It does not test everything. PGT-A does not check for every genetic condition.
• It is debated for routine use. For younger women without a history of loss or failure, high-quality studies have not shown a consistent advantage, and some women may lose usable embryos to a false "abnormal" result. Many specialists reserve PGT-A for selected patients.
• Cost and effort. It adds laboratory costs and usually means a frozen transfer. Costs depend on how many embryos are tested, and vary between clinics and laboratories, so please ask for a written estimate. Read about the IVF cost in Chennai.

The Law in India: No Sex Selection

Under the Pre-Conception and Pre-Natal Diagnostic Techniques (PCPNDT) Act, 1994, it is illegal to determine or disclose the sex of an embryo or foetus for non-medical reasons, and the same applies to PGT. Genetic laboratories and clinics may not reveal the sex of an embryo. Assisted reproductive technology is also regulated under the ART (Regulation) Act, 2021. PGT is used only for medical reasons: to improve the chance of a healthy pregnancy and to avoid serious genetic conditions.

Should You Have PGT-A?

I discuss it with you in the context of your age, your history, the number of embryos you are likely to have and your preferences. For some couples it makes sense; for many it does not. If you are not sure whether IVF itself is the right next step, read do you need IVF or can alternatives come first?. You can read more on my PGT genetic testing and IVF pages, or book a consultation with Dr. Rukkayal Fathima in Chennai.

Medical disclaimer: this article is general information and not medical advice. Rates quoted are approximate and depend on age and laboratory. Outcomes cannot be guaranteed. Please consult a qualified fertility specialist and genetic counsellor.