Preimplantation genetic testing (PGT), also known as embryo biopsy, is used to identify chromosomal or genetic defects in embryos prior to an in vitro fertilization (IVF) cycle. Genetic defects and chromosomal abnormalities are primary causes of both embryo implantation failure and miscarriage, making PGT a particularly powerful assisted reproduction technology for many aspiring parents.
There are different types of PGT, each designed to test for specific conditions. Two terms previously used for these procedures are preimplantation genetic diagnosis and preimplantation genetic screening – now referred to as PGT-M and PGT-A:
PGT-A screens embryos for the correct number of chromosomes, known as euploidy. Embryos with too many or too few chromosomes (aneuploidy) may result in conditions such as Down syndrome, Turner syndrome, or Klinefelter syndrome. It is recommended to all patients.
PGT-M is used when one or both parents are carriers of a known single-gene disorder, such as muscular dystrophy, cystic fibrosis, or Tay-Sachs disease. Embryos found to carry the disease-causing gene are not selected for transfer.
For individuals or couples with a known risk of passing on a genetic condition (often discovered through detailed family history evaluation by a trained geneticist), genetic testing can lower the risk of passing that disorder on to their child. Genetic testing can also help increase the chances of a healthy pregnancy by screening for potential genetic problems that might negatively affect pregnancy and birth.
The role of chromosomes in reproduction
Chromosomes, which contain DNA (deoxyribonucleic acid), come in pairs. A normal human cell contains 46 chromosomes, 23 pairs. Sometimes DNA contains an abnormal number (aneuploidy) or structural abnormality of chromosomes. In addition to aneuploidy, which is the most common abnormality, mutations can occur in the DNA sequence, also resulting in a genetic defect.
These structural abnormalities include translocations, inversions, and deletions of certain chromosomes. Both types of chromosome abnormalities can be present in a woman’s egg or a man’s sperm. The transmission of a chromosome abnormality to an embryo can result in implantation failure, miscarriage, pregnancy complications, or a baby born with a genetic disorder. Such a condition can cause various disabilities, health complications, and death.
PGT identifies whether an embryo is chromosomally normal before implantation. While PGT cannot eliminate all risk, genetic testing provides tremendous relief to parents and dramatically decreases the chances of an affected pregnancy.
The PGT-M Process
The PGT-M process begins by obtaining embryos developed in an IVF cycle. Once embryos have been retrieved, one or two cells are biopsied from the embryo. This procedure involves removing either a polar body, which is half of the maternal DNA, or a blastomere, which is a cell from the embryo that contains its DNA.
Once cells have been extracted, lab procedures are performed to test for abnormalities. The two most common techniques are fluorescent in situ hybridization (FISH) and polymerase chain reaction (PCR). PCR is used to detect single-gene (monogenetic) disorders. FISH is used to evaluate chromosome numbers and structures to identify the absence or presence of specific disorders in a developing embryo. As a result, only embryos free of genetic diseases are transferred to the mother’s uterus during the next stage of the IVF cycle. This increases the chance of conception and ultimately a healthy baby.
Why Use PGT-M?
PGT-M is most commonly suggested for couples who are aware that they are carriers, or potential carriers, of a certain genetic disorder. These include:
Mutations in BRCA1 (a cancer-causing gene).
Sickle cell disease.
Cystic fibrosis.
Huntington’s disease.
Tay-Sachs disease.
Muscular dystrophy and myotonic dystrophy.
Fragile X syndrome.
Doctors also recommend that women who are considered to be at advanced maternal age (35 and older) use PGT-M. As a woman gets older, the chromosomes in her eggs do not always divide normally, which can increase the chance of developing embryos with abnormal chromosomes. This can lead to a number of different genetic disorders, including BRCA1 which can lead to cancer.
PGT-A is performed very similarly to PGT-M. The process starts by gathering embryos created in an IVF cycle. Once embryos are retrieved, cells are biopsied from the embryos. This procedure involves removing either a polar body or a blastomere. Once cells have been extracted, the number of chromosomes present in each cell is evaluated. If a cell has 23 chromosomes from each parent (46 total), it is normal.
In evaluating for proper chromosome numbers, PGT-A tests for many chromosome problems in one test. Common conditions caused by chromosome abnormalities include Down syndrome, Turner Syndrome, and Klinefelter syndrome.
Why Use PGT-A?
Individuals and couples should consider PGT-A if they carry chromosomal abnormalities that may cause genetic conditions, which is generally determined through detailed family history evaluation by a trained geneticist. As a result, only embryos free of genetic diseases are transferred to the mother’s uterus during the next stage of the IVF cycle. This increases the chance of conception and ultimately of a healthy baby.
Much like PGT-M, PGT-A is also recommended for women ages 35 and older because women of advanced maternal age may produce eggs with aneuploidy, which is an abnormal number of chromosomes.
Risks and Considerations of PGT
PGT is relatively risk-free; however, with any form of technology, it is important to be cautious. In some cases, embryos can be diagnosed incorrectly. However, the chance of transferring a genetically defective embryo that was mistakenly identified as normal is only between 2 and 11%, depending on the specific disorder. In rare cases, the genetic testing biopsy process can damage embryos.
There are no reports of identifiable problems in babies born from IVF with PGT-M. But it is important to consider the possibility of abnormalities occurring later in life as a result of unknown damage that occurred during the genetic testing process.
Explore Your Genetic Testing Options in New Jersey
If you’re ready to explore PGT or other fertility treatments to fulfill your family-building dreams, take the first step and connect with the Reproductive Science Center of New Jersey. Using the most cutting-edge reproductive technology and compassionate care, we’ve helped countless individuals and couples from Brick, Hamilton, Princeton, Red Bank, and all of NJ expand their families. Contact us today for more information about PGT or to schedule an appointment at one of our fertility clinics in Eatontown, Toms River, or Lawrenceville.
