Preimplantation genetic testing at a glance
- Preimplantation genetic testing (PGT) evaluates embryos created during in vitro fertilization (IVF) for abnormalities in chromosomes and genes that can cause a failed implantation, miscarriage and/or genetic disease.
- PGT is the new medical term that replaces the former terms preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS), but the testing procedures are the same.
- Cystic fibrosis, muscular dystrophy, Tay-Sachs disease and sickle cell anemia are examples of inherited, single-gene conditions that can be identified by PGT.
- PGT also detects genetic defects related to a missing or extra chromosome in a pair (called aneuploidy) that can cause birth defects such as Down syndrome; PGT also can detect the rearrangement of genes in a chromosome that can result in a failed pregnancy or birth.
- Embryologists use this testing to screen for genetic defects in embryos during IVF, so only embryos with a negative result will be transferred to achieve a pregnancy.
Preimplantation genetic testing, formerly preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS)
The fertility medicine field has changed the terminology for the names of genetic tests we can perform on embryos created through IVF to identify the genetically healthy ones prior to transferring them to the woman’s uterus for pregnancy. The former tests known as preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are now called preimplantation genetic testing (PGT).
All embryos are susceptible to genetic defects, but we can only test embryos that are created in the lab via IVF. Depending on the age of the expectant mother, approximately 25%-85% of embryos will have chromosomal abnormalities. Others may have single-gene defects.
These embryonic chromosomal abnormalities and gene defects are more likely to cause implantation failure, miscarriage or a birth defect in a resulting child. Abnormalities are actually relatively common in natural conception and a significant factor in why 20% of all pregnancies end in miscarriage.
Genetic testing of embryos is a powerful technology available to those undergoing IVF. To perform these tests, a small number of cells (usually five or less) are taken from an embryo in a process called a biopsy.
A laboratory then evaluates the genetic makeup of these cells to determine which embryos are chromosomally normal or are not affected with genetic disease. Only those embryos determined to be healthy are eligible to be transferred into the uterus to establish a pregnancy.
The three types of PGT used in IVF: PGT-M, PGT-A & PGT-SR
PGT-M stands for preimplantation genetic testing for monogenetic (single), is used to identify disorders caused by a defect in a single gene, such as cystic fibrosis. That testing was formerly known as preimplantation genetic diagnosis (PGD). Most frequently diagnosed diseases and disorders detected by PGT-M follow.
- Cystic fibrosis.
- Sickle cell anemia.
- Spinal muscular atrophy.
- Tay-Sachs disease.
- Huntington’s disease.
- Charcot-Marie tooth disease.
- Fragile X syndrome.
- Duchenne muscular dystrophy.
PGT-A evaluates an embryo for the proper number of chromosomes in a pair (2). When one is missing or there is an extra chromosome, this is known as aneuploidy (the A in PGT-A). Down syndrome, which is an extra chromosome (called trisomy) in the chromosome pair # 21, is the most common form of aneuploidy. PGT-A used to be called preimplantation genetic screening (PGS). The most common abnormalities found by PGT-A include:
- Down syndrome.
- Turner syndrome.
- Patau syndrome.
PGT-SR evaluates an embryo for structural rearrangements (SR) of genetic material in a chromosome. These structural inversions or translocations result in embryos without the right amount of chromosome material, reducing the chance of a live birth and increasing the risk of recurrent miscarriage. The former PGS testing accomplished the same evaluation as PGT-SR. PGT-SR identifies these conditions:
- Robertsonian translocations.
- Reciprocal translocations.
- Nonreciprocal translocations.
Who should consider preimplantation genetic testing?
Preimplantation genetic testing of embryos is a valuable adjunct to the process of IVF. Because of its ability to improve IVF success rate, PGT is a good reproductive option for couples in three general categories.
- Individuals or couples who are carriers for an inheritable genetic disease (e.g. cystic fibrosis), for whom the risk of producing a child with an incurable genetic disease is 25% to 50%.
- Couples with an increased risk for producing abnormal embryos with an abnormal number of chromosomes. This includes couples and women who have had recurrent miscarriages, a previous aneuploidy pregnancy, failed embryo implantation, previous unsuccessful infertility treatments, or unexplained infertility.
- Couples in which one of the partners is a carrier for a chromosome rearrangement, such as a balanced translocation or inversion. They are at increased risk to produce chromosome abnormalities in their embryos resulting in increased rates of miscarriages and fetal abnormalities. PGT has been shown to improve outcomes in pregnancies conceived from carriers of balanced chromosome rearrangements.
Other options for those with genetic or chromosome abnormality risks
Besides PGT, other reproductive options available for couples with increased genetic and chromosomal reproductive risks are the use of donor eggs or donor sperm and adoption. After pregnancy is established, amniocentesis, in which a needle extracts amniotic fluid that contains fetal tissue, can test for chromosomal abnormalities as well as fetal infections. If the result shows a serious problem, the couple may need to have an elective second trimester termination.
PGT is safer than elective second trimester termination and psychologically more acceptable than elective termination. It provides couples at increased risk for producing children with incurable genetic diseases or chromosomal disorders more reproductive options than donor eggs or donor sperm, adoption and sterilization.
There are no documented health risks for children born after preimplantation genetic testing beyond the normal health risks to mother and child through IVF. Handling of the embryo, its biopsy, freezing and thawing result in a small risk of damage leading to an embryo that does not implant.
Another risk of PGT is inaccuracy in test findings, as the testing is not 100% accurate. For this reason, it is recommended that the patient undergo typical prenatal testing when she is pregnant, such as amniocentesis discussed above.
Couple has a baby with IVF and PGT after seven years of fertility treatments
Fertility Success Rates
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