PGT of embryos: the different types, and where the field is heading

When patients hear the term PGT for the first time, many ask the same question: "Can't we just transfer an embryo and hope for the best?" We can. But since 1990, reproductive medicine has learned to look inside an embryo before transfer, and that has changed how we treat infertility for good.

---

What PGT actually is

PGT, preimplantation genetic testing, is an analysis of cells from an embryo created during an IVF cycle, performed before it is transferred to the uterus. The biopsy is usually done on day 5–6 of development, once the embryo has reached the blastocyst stage, and the cells are taken from the trophectoderm: the outer layer that later forms the placenta, not the future fetus itself. This is a point I always explain to patients. The embryo doesn't "feel" the procedure, and at the blastocyst stage removing these cells has not been shown to reduce its chance of implanting. It is not a zero-risk step, because the biopsy can damage an embryo and a viable embryo can occasionally be set aside on an inaccurate result, but it is far gentler than the day-3 biopsy it replaced.

---

The main types of PGT

• PGT-A, for aneuploidy. Checks the number of chromosomes in the embryo. An extra or missing chromosome is the single most common cause of failed implantation and miscarriage, and the risk rises with a woman's age.

• PGT-M, for monogenic conditions. Used when one or both partners carry a known genetic mutation, for example cystic fibrosis or spinal muscular atrophy, and there is a risk of passing it on to a child.

• PGT-SR, for structural rearrangements. Used when one partner carries a balanced chromosomal translocation. The carrier is healthy, but some of their embryos may end up with an unbalanced set of chromosomes.

Occasionally this list also includes embryo Rh-factor determination and HLA typing, which matters when a future child could become a stem-cell donor for an older sibling with a serious illness.

---

A few facts that still surprise colleagues

• The first birth following this kind of testing dates back to 1990. At the time, doctors could only determine an embryo's sex, by amplifying a Y-chromosome-specific sequence; they removed just one or two cells for the analysis, and the biopsy was taken on day 3 of development, from a far more fragile stage of the embryo.

• The earliest laboratory method was FISH, which could assess only a limited number of chromosomes. Modern NGS, next-generation sequencing, reads all 24 chromosome types (the 22 autosomes plus X and Y) in a single run.

• Biopsies are now taken on day 5–6 rather than day 3, once the embryo has grown well past its earliest, most fragile stage. This has reduced the impact on the embryo and improved the accuracy of results.

• Registry data suggest PGT-A may reduce the risk of miscarriage, particularly in older patients, but ASRM considers the benefit unclear. It does not raise the overall chance of a live birth. By identifying a euploid embryo, it can make transferring a single embryo an easier decision, and single-embryo transfer is what avoids twins.

---

Where the field is heading

To me, the most interesting part of PGT isn't what already works but what is happening in labs right now.

Non-invasive PGT (niPGT-A). Instead of biopsying the trophectoderm, researchers are learning to read embryonic DNA directly from the culture medium the embryo grows in, because embryos naturally release a small amount of genetic material into it. The method is still experimental and not yet as accurate as a classic biopsy, but the direction is promising: if the technology matures, it would remove even the minimal risk associated with taking cells.

Artificial intelligence in embryo assessment. Algorithms trained on thousands of time-lapse images of embryo development can score embryos for the likelihood of implantation without any additional intervention, simply by analyzing morphology and the timing of cell division. So far, though, the evidence is that they match rather than beat a trained embryologist: the largest randomized trial, across 14 clinics, could not show the algorithm was even non-inferior to manual assessment. This doesn't replace genetic testing, and for now it remains an additional layer of selection whose value is still being established.

PGT-P (polygenic screening), the most controversial topic. Unlike PGT-A or PGT-M, which look for specific, defined problems, polygenic screening tries to estimate probabilistic risk across multifactorial traits: height, certain characteristics, the risk of some common diseases. The scientific community remains cautious. The predictive power of these tests at the level of a single embryo is still very limited, and the ethical questions are enormous. It is no accident that the UK, France, and Germany tightly regulate embryo testing itself, under laws written well before polygenic screening existed and never with it specifically in mind, which leaves little room for this kind of screening to be offered, while some private companies in the US continue to offer it. This debate will only get louder in the coming years, and I think it matters that patients understand the difference between "checking an embryo's health" and "selecting for desired traits". Those are two fundamentally different conversations.

---

What I tell my patients

PGT isn't about creating a "perfect" child. At its best it is a tool that helps some patients avoid transfers that were never going to work, and eases the emotional toll of repeated loss. It does not shorten treatment for everyone: the testing itself takes time, and for many patients the road to a pregnancy is no shorter. Like any tool, it isn't universal and doesn't replace a conversation with your doctor; for some patients it is a major advantage, for others it simply isn't necessary. But one thing is certain: this field is moving faster than almost any other in reproductive medicine, and what looks experimental today may well be standard practice within a few years.

---

This article is educational, not medical advice. Whether preimplantation genetic testing makes sense for you depends on your age, your diagnosis, and how many embryos you have; discuss it with your reproductive endocrinologist and, where a hereditary condition is involved, with a genetic counselor.