Preimplantation genetic diagnosis is a form of genetic testing performed on embryos created through IVF, designed to identify specific inherited conditions before the embryo is transferred to the uterus. Now formally called preimplantation genetic testing for monogenic disorders (PGT-M), the procedure allows prospective parents who carry genes for serious diseases to select embryos unaffected by those conditions. It’s distinct from broader chromosome screening and is tailored to families with a known genetic risk.
How PGT-M Differs From Other Genetic Tests
The term “preimplantation genetic diagnosis” has largely been replaced in clinical settings by more specific names that reflect what’s actually being tested. PGT-M targets single-gene disorders, meaning conditions caused by a mutation in one specific gene. This is what most people mean when they search for preimplantation genetic diagnosis.
Two related tests exist under the same umbrella. PGT-A (preimplantation genetic testing for aneuploidy) screens embryos for the wrong number of chromosomes, which is the leading cause of miscarriage and implantation failure. PGT-SR checks for structural chromosomal rearrangements, where the embryo has the right number of chromosomes but pieces are flipped, swapped between chromosomes, deleted, or inserted in the wrong location. Each test answers a different question, and some families may need more than one.
Conditions It Can Detect
PGT-M is used when one or both parents carry a known genetic mutation that could be passed to their children. The conditions tested fall into several categories depending on how the gene is inherited.
- Autosomal dominant conditions (one copy of the mutation is enough to cause disease): Huntington’s disease, neurofibromatosis type 1.
- Autosomal recessive conditions (the child needs two copies, one from each parent): cystic fibrosis, sickle cell anemia, Tay-Sachs disease.
- X-linked recessive conditions (carried on the X chromosome, primarily affecting males): hemophilia, fragile X syndrome, most neuromuscular dystrophies.
- X-linked dominant conditions: Rett syndrome, incontinentia pigmenti.
The ASRM Ethics Committee considers PGT-M ethically appropriate for adult-onset conditions as well, whether the mutation guarantees disease development or simply raises the risk significantly. This means families with hereditary cancer syndromes like BRCA mutations or conditions like Huntington’s, which may not appear until midlife, can use the technology.
How the Biopsy Works
All forms of preimplantation genetic testing require removing a small number of cells from the embryo. The standard approach today is a trophectoderm biopsy, performed when the embryo reaches the blastocyst stage, typically between day 5 and day 7 after fertilization. At this point the embryo has differentiated into two cell types: an inner cell mass (which becomes the fetus) and an outer layer called the trophectoderm (which becomes the placenta). The biopsy takes three to eight cells from the outer layer, leaving the inner cell mass untouched.
This replaced an older technique called cleavage-stage biopsy, which removed a single cell from the embryo on day 3. Though taking fewer cells sounds less invasive, the opposite turned out to be true. Removing one cell from an eight-cell embryo is proportionally a much bigger loss than removing a few cells from a blastocyst containing over a hundred. Research published in Zygote found that nearly 25% of chromosomally normal embryos biopsied at the cleavage stage failed to develop further, and the procedure significantly impaired implantation potential. The shift to trophectoderm biopsy improved cumulative live birth rates from about 34% to nearly 45% per cycle in the same study, while also providing more accurate results.
What Happens in the Lab
Once cells are removed, they’re sent to a genetics laboratory for analysis. The current gold standard is next-generation sequencing (NGS), a high-throughput method that reads DNA with enough resolution to detect whole and partial chromosomal abnormalities, mosaicism (where some cells in the biopsy are normal and others aren’t), and specific gene mutations. Older methods like array-based screening and quantitative PCR are still in use at some labs but detect fewer abnormalities. NGS flags more embryos as unsuitable for transfer than older platforms, but among patients who have at least one normal embryo available, clinical outcomes are comparable regardless of which technology was used.
Because testing takes time, embryos are frozen after biopsy while results are processed. Transfer happens in a subsequent cycle once results are back, a process called a frozen embryo transfer.
The Mosaicism Question
One of the more complicated aspects of genetic testing is mosaicism, where a biopsy shows a mix of normal and abnormal cells. This creates uncertainty: the biopsy samples only the outer layer, and those few cells may not represent the entire embryo. Adding to the confusion, the same biopsy sample could be classified as “high-level mosaic” at one lab, “aneuploid” at another, and “euploid” at a third, depending on the lab’s technology, thresholds, and reporting practices.
The ASRM recommends that each IVF program develop its own policies for how to handle mosaic embryos, and that clinicians use current evidence to decide whether to report mosaicism results at all. For patients, this means that if you receive a mosaic result, the conversation about what it means and whether that embryo could still be transferred will depend heavily on your clinic and their interpretation of the data.
Cost and What to Expect
PGT-M adds a layer of cost on top of standard IVF. The genetic testing itself typically runs $2,000 to $3,000, with many labs charging per embryo or in tiered pricing structures. More embryos tested means a higher bill. This does not include the cost of the IVF cycle that produced the embryos or the frozen embryo transfer that follows.
There’s also a step many people don’t anticipate: before PGT-M can be performed, the genetics lab needs to build a custom test (called a probe) specific to your family’s mutation. This requires DNA samples from the carrier parent and sometimes additional family members, and it can take several weeks to complete before your IVF cycle even begins. Planning ahead is essential, because the probe development timeline can delay your treatment if you aren’t aware of it.
Ethical Boundaries and Limits
PGT-M for serious genetic diseases is broadly accepted by major reproductive medicine organizations. The more contested territory involves newer technology called polygenic embryo screening (PGT-P), which attempts to predict an embryo’s risk for complex traits influenced by many genes, like heart disease, diabetes, or even height and intelligence. The ASRM Ethics and Practice Committees have concluded that polygenic screening is not ready for clinical use, calling it “a nascent and unproven technology” whose predictive accuracy, safety, and clinical value are not supported by current evidence. They recommend that any use of PGT-P occur only in research settings under institutional review board oversight until the scientific, clinical, and ethical questions are resolved.
The distinction matters because PGT-M tests for a single, well-understood mutation with a clear inheritance pattern, while PGT-P tries to predict outcomes shaped by hundreds or thousands of genetic variants interacting with environment and lifestyle. The accuracy gap between the two is enormous, and the ethical implications of selecting embryos based on complex trait predictions remain unresolved.