What genetic tests are available for IVF?
Preimplantation Genetic Testing (PGT) in IVF involves the use of genetic screening techniques to evaluate the chromosomal makeup of an embryo prior to implantation. Couples may choose to undergo genetic testing for many reasons. Some may choose this route following a loss, as many miscarriages are due to chromosomal abnormalities. Others may choose PGT due to a specific or known pre-dispositions with the racial or ethnic background of the couple, their family’s medical history, and associated conditions or genetic factors. Couples using a gestational carrier may choose PGT as an additional screening tool and to optimize single embryo transfer.
For patients navigating infertility, genetic tests can be broadly categorized into two types: preconception genetic testing (testing the people that are planning to conceive typically before trying and before IVF), and preimplantation genetic testing (testing the embryos as a part of conceiving with IVF).
Preconception Genetic Testing (Testing the parents): Pre-pregnancy genetic screening may help identify couples who have an increased risk of age-related or familial genetic disorders and birth defects. No single test, however, can accurately predict the risk of all defects in a child, and many birth defects, such as those related to environmental and toxic exposures and those that are random and unexplained, are not genetically based and may not be detected with genetic screening.
The two types of genetic testing on patients:
● Chromosomal Karyotype: A karyotype is a diagnostic blood test that looks at the size, shape, and number of your chromosomes, which contain your genes. Genes are parts of DNA passed to children from the egg provider and sperm provider. They carry information that determines your unique traits, such as height and eye colors, as well as risk for genetic disease. People typically have 46 chromosomes, divided into 23 pairs. One chromosome in each pair comes from the egg and the other from the sperm.
● Genetic Carrier Screening/Expanded Carrier Screening: Genetic carrier screening is a test typically run on DNA extracted from a saliva or blood sample. The goal is to determine if a patient carries a genetic condition that could be passed to their children. There are two main ways that genetic diseases are inherited. Autosomal recessive inheritance is the type of genetic disease that can occur in a baby if both sperm and egg provider carry for the same disease. X-linked inheritance is the type of disease that travels on the X-chromosome, and only the egg or the sperm provider need to carry in order to pass it on to offspring. Screening for these diseases ahead of time can reveal increased reproductive risks.
The American College of OBGYN (ACOG) and American College of Medical Genetics (ACMG) recommend that all patients be offered carrier screening. Depending on ancestral background and family history, these screenings can include high risk disorders such as Spinal Muscular Atrophy, Cystic Fibrosis, Tay Sachs, Fragile X and Hemoglobinopathies.
Preimplantation Genetic Testing (Testing the embryos): There are 3 recognized categories of preimplantation genetic testing, or “PGT”. The current terms as defined by ASRM are:
● PGT-A: Preimplantation Genetic Testing for Aneuploidy: PGT-A can help to answer the question of which embryos have the greatest potential to become a healthy pregnancy and baby. Aneuploidy means that PGT found embryos have missing or extra chromosomes or pieces of chromosomes. A normal, or “euploid,” embryo has 46 chromosomes: 23 from the sperm and 23 from the egg. “Aneuploid” embryos have missing or extra chromosomes and are unlikely to implant, and/or likely to result in pregnancy loss.
● PGT-SR: Preimplantation Genetic Testing for Structural Rearrangement: Similar to PGT-A, this test also checks for aneuploidy in embryos. However, it’s more typically ordered when there is a known “structural rearrangement” in the sperm or egg provider. A structural rearrangement is an umbrella term for specific kinds of chromosomal abnormalities. PGT-SR is a more specific type of PGT-A which looks a little closer at the chromosomes involved. It reduces the risk of having a pregnancy or child with an unbalanced structural abnormality, which involves extra or missing genetic material and typically results in a miscarriage.
● PGT-M: Preimplantation Genetic Testing for Monogenic disease: PGT-M is used to help patients reduce their risk of having a child born with a known inherited disorder caused by mutations in a single gene (“monogenic”), such as cystic fibrosis, BRCA1, or Huntington’s disease. Before PGT-M can be performed, there is typically a “probe” development process that can take several weeks. PGT-M test development typically relies on mapping specific genetic variants, and is custom made for each patient/couple based on the disease they are testing. The goal of PGT-M is to identify which embryos are affected with disease so that unaffected embryos can be selected for transfer.
Article Sources: Pacific NW Fertility; New Hope Fertility
