Preimplantation genetic testing (PGT) is a laboratory technique used alongside IVF to examine the genetic makeup of embryos before they are transferred to the uterus. By identifying chromosomal or genetic abnormalities early, PGT helps intended parents and their IVF doctor choose embryos with the best chance of resulting in a healthy pregnancy. There are three main types of PGT, each designed for a different purpose: PGT-A, PGT-M, and PGT-SR.
What Is Preimplantation Genetic Testing?
During an IVF cycle, embryos are grown in a laboratory for five to six days until they reach the blastocyst stage. At this point, a small number of cells are carefully removed from the outer layer of the embryo (the part that will eventually form the placenta, not the fetus itself) in a process called a trophectoderm biopsy. These cells are sent to a genetics lab for analysis while the embryo is frozen and stored at the IVF centre, awaiting results.
Once testing is complete, the embryology and genetics team can identify which embryos are chromosomally normal or free of a specific inherited condition, allowing for more informed embryo selection during a future frozen embryo transfer.
PGT-A: Testing for Aneuploidy
PGT-A (Preimplantation Genetic Testing for Aneuploidy) screens embryos for an abnormal number of chromosomes. A normal human embryo should have 46 chromosomes 23 from each parent. Aneuploidy occurs when there are extra or missing chromosomes, a common cause of failed implantation, miscarriage, and conditions such as Down syndrome (Trisomy 21).
Who might consider PGT-A:
- Women of advanced maternal age (typically 35 and older), since egg quality and chromosomal accuracy decline with age
- Couples with a history of recurrent miscarriage
- Those who have had previous failed IVF cycles
- Couples using IVF who want to reduce the chance of transferring a chromosomally abnormal embryo
What it can tell you: PGT-A does not diagnose specific diseases. Instead, it identifies whether an embryo has the correct number of chromosomes (euploid), too many or too few (aneuploid), or a mix of both (mosaic). Only euploid or, in some cases, low-level mosaic embryos are typically prioritized for transfer.
PGT-M: Testing for Monogenic (Single-Gene) Disorders
PGT-M (Preimplantation Genetic Testing for Monogenic disorders), formerly known as PGD (Preimplantation Genetic Diagnosis), is used when one or both parents carry a known genetic mutation for a specific inherited condition. This test looks for that particular mutation in the embryo’s DNA.
Conditions commonly tested for with PGT-M include:
- Cystic fibrosis
- Huntington’s disease
- Sickle cell disease
- Tay-Sachs disease
- BRCA1/BRCA2 mutations (hereditary breast and ovarian cancer risk)
- Spinal muscular atrophy
Who might consider PGT-M:
- Couples who are known carriers of a single-gene disorder
- Individuals with a family history of a serious inherited condition
- Couples who have previously had a child affected by a genetic disease
Before pursuing PGT-M, a custom genetic probe must usually be developed for the specific mutation in question, which requires genetic counseling and sometimes additional testing of family members. Your IVF doctor will typically coordinate closely with a genetic counselor throughout this process to make sure the probe is accurate before testing begins.
PGT-SR: Testing for Structural Rearrangements
PGT-SR (Preimplantation Genetic Testing for Structural Rearrangements) is used when one or both parents carry a chromosomal rearrangement, such as a translocation or inversion. In these cases, the total genetic material may be normal, but it is arranged differently, which can lead embryos to inherit unbalanced amounts of chromosomal material.
Who might consider PGT-SR:
- Individuals known to carry a balanced translocation or inversion
- Couples with a history of recurrent pregnancy loss linked to a structural chromosomal issue
- Those identified as carriers through prior genetic testing, often after a difficult pregnancy history
What it can tell you: PGT-SR helps identify embryos with a balanced or normal chromosomal structure, reducing the risk of implantation failure or miscarriage caused by unbalanced chromosomal material.
How the Testing Process Works
- Ovarian stimulation and egg retrieval — as in standard IVF
- Fertilization — typically using ICSI (intracytoplasmic sperm injection) to minimize contamination of the sample with extra sperm DNA
- Embryo culture — embryos are grown to the blastocyst stage (Day 5, 6, or 7)
- Biopsy — a few cells are removed from the trophectoderm
- Vitrification (freezing) — the embryo is frozen while biopsy results are pending
- Genetic analysis — the biopsied cells are analyzed in a specialized genetics lab
- Results and embryo selection — the clinical team reviews results with the patient to select the embryo(s) for transfer
- Frozen embryo transfer (FET) — the chosen embryo is thawed and transferred in a subsequent cycle
Benefits and Limitations
Potential benefits:
- May reduce the risk of miscarriage related to chromosomal abnormalities
- May improve the chances of a successful pregnancy per embryo transfer
- Can lower the likelihood of transferring an embryo affected by a known inherited condition
- May help reduce the emotional and physical toll of repeated failed transfers in some patients
Limitations to consider:
- PGT adds cost and time to the IVF process
- Biopsy carries a small theoretical risk to the embryo, though modern techniques have made this minimal
- Testing cannot guarantee a healthy baby or successful pregnancy it reduces certain risks but does not eliminate them
- Mosaic results can be complex and may require detailed genetic counseling to interpret
- Not all genetic or developmental conditions can be detected through PGT
Is PGT Right for You?
The decision to pursue PGT-A, PGT-M, or PGT-SR is highly individual and depends on factors such as age, reproductive history, known genetic risks, and personal values. Genetic counseling is a valuable and often essential part of this decision-making process, helping patients understand test accuracy, potential outcomes, and how results might affect their family planning.
Anyone considering PGT should discuss their specific situation with a reproductive endocrinologist and a certified genetic counselor to determine whether testing aligns with their medical history and goals. Choosing an experienced IVF centre with an accredited genetics lab and a skilled embryology team can also make a meaningful difference in the accuracy and reliability of your results.