Preimplantation Genetic Testing (PGT) provides patients with the option to screen in vitro fertilization (IVF) created blastocyst stage embryos for genetic or chromosomal abnormalities. Screened embryos that do not have abnormalities have a higher likelihood of uterine implantation and pregnancy success. For those who want to know the gender, these tests can also identify the sex of the embryos by examining the sample for X and Y chromosomes.
How Does Testing Work?
The resulting embryos created in the IVF lab can be biopsied with the removal of a few cells from each embryo. Biopsy is performed on blastocysts, embryos that are at day 5 or 6 (or less commonly on Day 7) of embryonic development, and the biopsied blastocysts are cryopreserved as biopsy results are awaited. The DNA from the isolated cell sample is used to check chromosome analysis and even can be used to screen for genetic conditions that are caused by mutation on a part of a chromosome. Results from the biopsy will provide information on which embryos are optimal for thaw and frozen embryo transfer since they have a higher chance of success.
What Genetic Conditions Can Be Screened For?
PGT screening can be performed for the following:
PGT-A (formerly PGS)
Preimplantation Genetic Testing for Aneuploidy, formerly known as PGS (Preimplantation Genetic Screening), detects the number of chromosomes in an embryo. A euploid embryo (normal embryo) each has 23 pairs of chromosomes, 46 total with half from the sperm and half from the egg. An aneuploid embryo contains missing or extra chromosomes. Aneuploid embryos frequently fail at implantation, are likely to miscarry and contribute to birth defects. By knowing the chromosome count of embryos, an embryo with the correct count can be chosen for transfer into the uterus to increase the chances of getting pregnant.
Patients may consider PGT-A that fall into one of the following categories:
- Advanced Maternal Age: Women who are 35 years or older are at a higher risk of having embryos with abnormal chromosomes, which can decrease embryo implantation rate, increase the likelihood of having miscarriages, and have children born with birth defects.
- Recurrent Pregnancy Loss: Couples who have experienced past miscarriages may benefit from identifying embryos with abnormal chromosomes.
- Repeated IVF Failures: For those who have had multiple unsuccessful IVF cycles, PGT-A can be used to identify viable embryos and improve the success rate of future IVF attempts.
- History of Aneuploidy: For those with a history of chromosomal irregularities or who have previously had a child with a chromosomal disorder, the risk of recurrence can be reduced with PGT-A testing.
- Unexplained Infertility: In cases of unexplained infertility, testing can help identify embryos with abnormal chromosomes that may contribute to infertility.
PGT-M (formerly PGD)
Preimplantation Genetic Testing for Monogenic/Single Gene Defects, formerly known as Preimplantation Genetic Diagnosis (PGD or PGT-D), is used to screen for known genetic mutations or diseases that can be passed on from prospective parents to their offspring. An embryo that is normal is likely to be free of the genetic condition tested for and would be considered for transfer into the uterus.
Individuals and couples who are at high risk of passing on inherited conditions may consider PGT-M, which includes:
- Autosomal Recessive Conditions: Couples who have the same autosomal recessive conditions such as cystic fibrosis, Tay-Sach disease, and sickle cell anemia
- Autosomal Dominant Disorders: Couples who have an autosomal dominant disorder such as Huntington’s disease, Marfan syndrome, and Achondroplasia
- X-linked Gene Mutations: Individuals who are carriers of x-linked gene mutations such as Duchenne Muscular Dystrophy
Those who had a child affected by a monogenic disorder should consider PGT-M.
Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR), looks for structural irregularities in the chromosomes such as translocations, inversions, deletions, or duplications of genetic material within the chromosomes. These rearrangements can lead to infertility, recurrent miscarriages, or the birth of children with developmental disabilities or other health issues. By weeding out which embryos are irregular, those with balanced or normal structural rearrangements can then be selected for implantation.
Individuals and couples who carry structural chromosomal rearrangements or those who have experienced difficulties with fertility or recurrent pregnancy loss associated with such rearrangements should consider PGT-SR.
PGT provides patients with information that can decrease genetic conditions, select gender, and increase embryo implantation rate to achieve pregnancy. Patients should discuss with their fertility specialists the different test options and how they could help with achieving a successful pregnancy.