PGT stands for preimplantation genetic testing, a group of lab techniques used during IVF to check embryos for genetic or chromosomal problems before they’re transferred to the uterus. The basic idea is straightforward: after eggs are fertilized in the lab, a few cells are removed from each embryo, analyzed, and the results help you and your doctor choose which embryo has the best chance of a healthy pregnancy. There are three types of PGT, each designed to look for something different.
The Three Types of PGT
PGT-A (aneuploidy) screens embryos for the wrong number of chromosomes. A healthy human cell has 46 chromosomes, organized in 23 pairs. When an embryo has an extra or missing chromosome (a condition called aneuploidy), it can lead to failed implantation, miscarriage, or conditions like Down syndrome. PGT-A checks all 24 chromosome types at once to flag embryos with these errors. This is the most commonly offered form of PGT and is sometimes recommended for people over 35 or those with a history of miscarriage.
PGT-M (monogenic disorders) looks for a specific inherited disease that runs in your family. It’s used when one or both partners carry a known genetic mutation for conditions like cystic fibrosis, sickle cell disease, Huntington’s disease, muscular dystrophy, fragile X syndrome, or hereditary cancer syndromes such as BRCA. Unlike PGT-A, which casts a wide net across all chromosomes, PGT-M zeroes in on a single gene. The goal is to select embryos that either don’t carry the disease-causing mutation or, in the case of recessive conditions, are healthy carriers who won’t develop the disease.
PGT-SR (structural rearrangements) is for couples where one partner has a known chromosomal rearrangement, meaning pieces of their chromosomes are swapped, flipped, deleted, or duplicated. These rearrangements often don’t cause symptoms in the person who carries them, but they can lead to embryos with unbalanced chromosomes, raising the risk of miscarriage or birth defects. PGT-SR identifies embryos that inherited a normal or balanced chromosome arrangement.
How the Biopsy Works
All three types of PGT require a small tissue sample from each embryo. This biopsy happens on day 5, 6, or occasionally day 7 after fertilization, when the embryo has reached what’s called the blastocyst stage. At this point, the embryo has two distinct groups of cells: an inner cluster that will become the baby and an outer layer (the trophectoderm) that will become the placenta. The biopsy targets only the outer layer, leaving the inner cell mass untouched.
During the procedure, an embryologist uses a laser to make a small opening in the embryo’s outer shell, then gently removes about 5 to 8 cells from the trophectoderm. Those cells are sent to a genetics lab for analysis. Because results take time, the embryos are frozen immediately after biopsy and stored until the report comes back.
What Happens After the Biopsy
Genetic results typically come back within 7 to 14 days, though complex cases or busy labs can take longer. Because of this waiting period, a fresh embryo transfer in the same cycle isn’t possible. Instead, your doctor will schedule a frozen embryo transfer in a following menstrual cycle, adding roughly 2 to 4 weeks to the overall IVF timeline. In total, an IVF cycle with PGT usually takes 6 to 8 weeks from start to transfer.
Results will classify each embryo into one of a few categories. For PGT-A, embryos are labeled as normal (correct chromosome count), abnormal, or mosaic. For PGT-M and PGT-SR, embryos are classified as affected, unaffected, or carrier depending on the specific condition being tested.
What a Mosaic Result Means
Sometimes PGT-A results land in a gray area. A mosaic embryo is one where the biopsied cells show a mix: some have the normal number of chromosomes and some don’t. This doesn’t necessarily mean the entire embryo is abnormal. Because the test analyzes DNA from a small group of cells collectively, it detects an intermediate signal rather than counting chromosomes cell by cell.
Mosaic results are genuinely tricky. Clinical outcomes for mosaic embryo transfers have been inconsistent across studies, and there’s no universal guideline on what to do with them. Some mosaic embryos can lead to healthy pregnancies. The American Society for Reproductive Medicine recommends that anyone considering the transfer of a mosaic embryo meet with a genetic counselor who specializes in PGT to talk through the specific findings and what they mean for that particular embryo.
Does PGT-A Improve Pregnancy Rates?
This is where it gets nuanced. PGT-A reliably identifies chromosomally normal embryos, and transferring a normal embryo does give a higher chance of implantation per individual transfer. But that doesn’t always translate to a higher chance of taking home a baby from a full IVF cycle, because some embryos labeled abnormal might have still led to healthy pregnancies, and testing discards them from the pool.
A large analysis using data from the Society for Assisted Reproductive Technology found that for women under 35, cumulative live birth rates were actually slightly lower with PGT-A (70.6%) than without it (71.1%). For women aged 35 to 37, there was no meaningful difference between the two groups. The biggest potential benefit appears in older patients, where the proportion of abnormal embryos is much higher and selecting a normal one may spare failed transfers and miscarriages.
On miscarriage specifically, some individual studies have shown striking reductions. One study in women aged 38 to 41 found a miscarriage rate of 2.7% with PGT-A compared to 39% without it. Pooled data across broader populations showed a more modest gap of about 9% versus 21%. However, the ASRM notes that the overall evidence for PGT-A reducing miscarriage risk remains unclear, with many studies not large enough to draw firm conclusions.
Who PGT Is Most Useful For
PGT-M and PGT-SR have clear, well-defined purposes. If you or your partner carry a known genetic mutation or chromosomal rearrangement, these tests directly reduce the chance of passing that specific condition to your child. The clinical value here is rarely debated.
PGT-A is more of a judgment call. It tends to be most helpful for people over 37 or 38 (when the rate of chromosomally abnormal embryos rises sharply), those who’ve had multiple miscarriages, or those who’ve experienced repeated failed embryo transfers. For younger patients with a good number of embryos, PGT-A may not improve cumulative outcomes and adds cost, typically ranging from $3,000 to $6,000 on top of standard IVF fees. It also adds several weeks to the process and requires that all embryos be frozen rather than transferred fresh.
PGT-M requires a custom setup before your IVF cycle begins. The genetics lab needs DNA samples from both partners (and sometimes other family members) to map the mutation and design a test specific to your family. This preclinical workup can take several weeks on its own, so it’s worth starting the conversation with your clinic early in the process.