PGT, or preimplantation genetic testing, is a group of genetic tests performed on embryos created through IVF before they are transferred to the uterus. The goal is to identify embryos with chromosomal or genetic abnormalities so that only healthy embryos are selected for pregnancy. There are three distinct types of PGT, each designed to detect a different category of genetic problem.
The Three Types of PGT
PGT isn’t a single test. It’s an umbrella term covering three separate assays, and the one you’d use depends entirely on what you’re screening for.
PGT-A (aneuploidy) is the most broadly used version. It screens embryos for abnormal numbers of chromosomes across all 22 pairs of autosomes plus the X and Y sex chromosomes. Aneuploidy, having too many or too few chromosomes, is a leading cause of implantation failure, miscarriage, and conditions like Down syndrome. PGT-A doesn’t look for a specific inherited disease. It’s a general screen.
PGT-M (monogenic disorders) targets a specific single-gene mutation that runs in a family. This is the test used when one or both parents carry a known pathogenic variant for conditions like cystic fibrosis, Huntington’s disease, sickle cell disease, Duchenne muscular dystrophy, fragile X syndrome, or hereditary cancer syndromes such as BRCA-related breast and ovarian cancer and Lynch syndrome. The most commonly tested conditions fall into three categories: autosomal recessive disorders (cystic fibrosis and hereditary hemoglobinopathies), autosomal dominant disorders (myotonic dystrophy type 1, neurofibromatosis, Huntington’s disease, hereditary cancers), and X-linked disorders (Duchenne muscular dystrophy, hemophilia, fragile X).
PGT-SR (structural rearrangements) is used when a parent has a known structural chromosomal abnormality, such as a translocation, inversion, deletion, or insertion. These rearrangements can cause embryos to inherit unbalanced chromosome segments, leading to failed implantation or pregnancy loss.
How the Biopsy Works
All three types of PGT require removing a small number of cells from the embryo for analysis. This biopsy can technically happen on day 3 of embryo development, when the embryo has just six to eight cells and one or two cells are removed. But the standard practice today is a day-5 biopsy, performed at the blastocyst stage when the embryo has grown to 80 to 100 cells.
At the blastocyst stage, the embryo has differentiated into two parts: the inner cell mass (which becomes the baby) and the trophectoderm (which becomes the placenta). The biopsy takes four to six cells from the trophectoderm only, leaving the inner cell mass untouched. A laser opens the outer shell of the embryo, and the cells are gently removed through micromanipulation. Because you’re sampling from a much larger pool of cells, a day-5 biopsy is less disruptive to the embryo than a day-3 biopsy and provides more genetic material to analyze.
What Happens After the Biopsy
Once the cells are removed, the embryo is immediately frozen through vitrification. It stays frozen while the biopsied cells are sent to a genetics lab for analysis. Results typically come back in one to two weeks, though complex chromosomal situations can take an additional two to four weeks.
If results show one or more embryos are genetically normal, you’ll proceed with a frozen embryo transfer in a subsequent cycle. Your doctor will thaw the selected embryo and transfer it to the uterus when conditions are right for implantation. This means PGT always adds at least one extra cycle to your IVF timeline compared to a fresh transfer.
The Technology Behind the Results
Modern PGT relies primarily on next-generation sequencing (NGS), which reads the genetic material from the biopsied cells and maps chromosomal content with high resolution. NGS can detect mosaicism (a mix of normal and abnormal cells in the same embryo) at levels as low as 20%, which older technologies like FISH couldn’t reliably catch. FISH, the original method used in the 1990s, only examined a handful of chromosomes and missed roughly 42% of abnormalities that newer platforms detect.
What Mosaic Results Mean
Not every result comes back as a clean “normal” or “abnormal.” Some embryos are mosaic, meaning the biopsy contains a mix of chromosomally normal and abnormal cells. This is actually common in human embryos at the blastocyst stage.
Based on guidelines from the Preimplantation Genetic Diagnosis International Society, embryos are classified on a sliding scale. Samples with less than 20% abnormal cells are classified as normal (euploid). Samples with more than 80% abnormal cells are classified as abnormal (aneuploid). Everything in between, the 20 to 80% range, is considered mosaic. Within that range, lower mosaicism carries lower risk and higher mosaicism carries higher risk.
Euploid embryos are always prioritized for transfer. When no euploid embryos are available, transferring a mosaic embryo with lower-level mosaicism can be considered after genetic counseling, though this decision involves careful risk assessment based on which specific chromosomes are affected and the degree of mosaicism.
Does PGT-A Improve Live Birth Rates?
This is where the evidence gets more nuanced than many fertility clinics let on. PGT-A clearly improves outcomes in specific scenarios: transferring a single tested embryo produces ongoing pregnancy rates comparable to transferring two untested embryos (about 61% versus 65%), while virtually eliminating the risk of twins. In one study, the multiple pregnancy rate dropped from 53% to 0% when single euploid embryos were transferred.
However, a large analysis of over 43,000 US IVF cycles reported to the CDC found that, overall, clinics with higher PGT-A utilization did not have better live birth rates. In fact, for all age groups combined, live birth rates were 36.8% at low-utilization clinics compared to 29.8% at high-utilization clinics. The decline was most pronounced in women under 35, where rates dropped from 50.6% to 45.6%. A separate analysis of over 133,000 US IVF cycles found similar patterns: PGT-A was associated with decreased cumulative live birth rates at all ages except above 40.
Where PGT-A does show a potential benefit is in reducing certain types of pregnancy loss. In women of advanced maternal age, one study found biochemical pregnancy losses dropped from 31.5% to 3.7% with PGT-A. Live birth rates per transfer, however, were not significantly higher in the tested group. The likely explanation is that PGT-A helps you avoid transferring embryos that would fail, but the biopsy process, the freezing, and the inevitable discarding of some embryos that might have been viable all carry their own costs to your cumulative chances.
Cost of PGT
PGT is an add-on to your IVF cycle and is not included in the base cost of treatment. Pricing varies widely depending on the lab, the number of embryos biopsied, and the type of testing. Published cost estimates for PGT-A, including both the biopsy procedure and the genetic analysis, range from roughly $3,000 to over $12,000 per cycle. PGT-M can cost more because it often requires a custom probe to be designed for your family’s specific mutation before the cycle even begins, which adds both time and expense. Most insurance plans do not cover PGT, though some cover it partially when there is a documented medical indication.
Who PGT Is Most Useful For
PGT-M and PGT-SR have the most clear-cut indications. If you carry a known genetic mutation or chromosomal rearrangement, these tests allow you to select embryos that don’t carry the condition, something that can’t be accomplished any other way before pregnancy.
PGT-A is more debated. The strongest case for it exists in women over 40, where aneuploidy rates in embryos are very high and the test helps avoid repeated failed transfers. It also makes sense when you want to confidently transfer a single embryo and avoid a twin pregnancy. For women under 35 with no history of genetic concerns or recurrent loss, the benefit is less established, and some evidence suggests it may reduce cumulative live birth chances by removing embryos that might have self-corrected or been mislabeled due to mosaicism in the biopsy sample.
The biopsy only samples the outer layer of the embryo, not the cells that become the baby. A mosaic or even aneuploid trophectoderm result doesn’t always reflect what’s happening in the inner cell mass. This biological limitation is one reason PGT-A results are considered a screening tool rather than a definitive diagnosis.