Should we have genetic testing?
What is Preimplantation Genetic Testing (PGT)?
Preimplantation genetic testing (PGT) is a technique in which one or more cells are taken from an egg or embryo (fertilized egg) for testing to provide information about the genetic make-up of the rest of the cells in that embryo. It is done in a lab, using in vitro fertilization (IVF). To learn more about IVF, please see the ASRM fact sheet titled In vitro fertilization (IVF). Genetically healthy embryos are transferred to the uterus, where they may attach to its lining and produce a pregnancy.
Why should we do Preimplantation Genetic Testing (PGT)?
Patients with many inherited familial diseases can have their embryos tested to determine its genetic make-up. Specifically, this would include patients with a history of single-gene disorders (such as cystic fibrosis or sickle cell anemia) and patients with a history of sex-linked disorders (such as Duchenne muscular dystrophy and Fragile X syndrome). In addition, even families in search of a bone marrow donor may be able to use PGT to bring a child into the world that can provide matching stem cells for an affected sibling.
What are the different types of Preimplantation Genetic Testing (PGT)?
- PGT- A (Aneuploidy)
- Checks for extra or missing chromosomes called aneuploidy. This test can find conditions like Down syndrome—an extra chromosome number 21. However, aneuploidy can occur with any of the chromosome pairs. Most cases of aneuploidy result in failed embryo transfers or miscarriages. Therefore, the biggest advantage of PGT-A is to identify embryos with a higher likelihood of resulting in a successful and healthy pregnancy.
Although aneuploidy is a potential testing result it isn't an inherited genetic trait but is instead related to the physical condition and quality of the biological mother's eggs. Aneuploidy is linked to age. By the time a woman is 35 years old, about half her embryos are expected to be aneuploid. By age 40, this increases to about 80%.
- PGT - M (Monogenic)
- Checks for single-gene mutations to reduce the risk of couples with a known inherited condition caused by mutations in a single gene such as Huntington disease, and early-onset Alzheimer disease; for cancer predisposition genes, such as BRCA mutations; and for nonfatal but potentially serious conditions that are apparent at birth, such as focal dermal hypoplasia.
- PGT- SR (Structural Rearrangements)
- Is used to detect unbalanced chromosome rearrangements. Remember that a normal or euploid human embryo has 46 chromosomes—23 pairs, one copy of each pair comes from each parent, including the numbered chromosomes one to twenty-two and the sex chromosomes X and Y.
What genetic tests can be performed?
Cells from the embryo can be tested for a gene that may produce a specific disease. This will show if an embryo is normal (no disease) or affected (has the disease). It will also show if the embryo is carrying the disease but unaffected by it. An unaffected carrier is someone who can pass the disease to his or her children but does not personally have any signs of the disease.
A karyotype is another kind of genetic test that can check the chromosomes. It is done during pregnancy, either by a chorionic villus sampling or amniocentesis. This test shows if the fetus has more or fewer chromosomes than the usual 46. Having too many or too few chromosomes can cause disorders such as Down syndrome (Trisomy 21) or Trisomy 13. This test can also look for out-of-order chromosomes, called translocations, which can cause problems with growth or function.
How is Preimplantation Genetic Testing (PGT) performed?
One or more cells are taken from an embryo and sent for genetic testing while the embryo is growing in the IVF laboratory. The testing can be done at different stages using different techniques. Techniques include fluorescent in situ hybridization (FISH), microarray, and single-nucleotide polymorphism (SNP).
Techniques are constantly evolving and changing to be more accurate, easier to run, and give more information. Newer techniques that include microarray and genome sequencing are more common methods of genetic testing.
Here's more information on PGT:
Preimplantation Genetic Testing
At this time, ASRM considers genetic screening for this indication experimental. If you are uncertain about genetic testing for you, speak with your physician about whether preimplantation genetic testing is right for you.