In Vitro Fertilization (IVF) offers a pathway to parenthood for many individuals and couples facing fertility challenges. Genetic testing of embryos, known as Preimplantation Genetic Testing (PGT), assesses embryos before uterine transfer to identify potential genetic issues. This article explores what these tests look for and their role in the IVF process.
Overview of Preimplantation Genetic Testing
Preimplantation Genetic Testing (PGT) is a specialized procedure used during an IVF cycle to screen embryos for genetic conditions before uterine transfer. PGT identifies embryos with genetic abnormalities, aiming to improve the chances of a successful pregnancy and reduce the risk of passing on certain genetic diseases.
This process involves biopsying 3-10 cells from the embryo’s outer layer (trophectoderm) at the blastocyst stage (day 5 or 6). These cells are sent for genetic analysis, while the embryo is frozen. PGT helps select embryos that are more likely to lead to a healthy pregnancy, enhancing IVF success rates.
Detecting Chromosomal Aneuploidies (PGT-A)
Preimplantation Genetic Testing for Aneuploidy (PGT-A) examines embryos for abnormal chromosome numbers. Aneuploidy, having too many or too few chromosomes (not the typical 46 in 23 pairs), frequently causes implantation failure, miscarriage, or genetic conditions. Its incidence increases with advanced maternal age.
PGT-A detects various aneuploidies, including trisomies (extra chromosome copies) like Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome). It also identifies monosomies (missing chromosomes), such as Monosomy X (Turner syndrome), and other sex chromosome aneuploidies like Klinefelter syndrome. Identifying these conditions is important as they can significantly impact embryo development, increase the risk of pregnancy loss, or result in a live birth with a genetic condition.
Identifying Single Gene Disorders (PGT-M)
Preimplantation Genetic Testing for Monogenic Disorders (PGT-M) is used when prospective parents carry a specific single gene disorder. These inherited disorders result from a single gene mutation. PGT-M identifies embryos that have inherited this mutation, allowing selection of unaffected embryos for transfer.
This testing is specific to the known familial mutation. Examples include Cystic Fibrosis, Huntington’s Disease, Sickle Cell Anemia, Tay-Sachs Disease, and Spinal Muscular Atrophy. PGT-M helps high-risk families reduce the chance of their child being affected by a genetic disorder.
Screening for Structural Chromosomal Rearrangements (PGT-SR)
Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR) is performed when one or both parents carry a structural chromosomal rearrangement. These involve changes like balanced translocations or inversions, where genetic material is rearranged without net gain or loss in the carrier parent. Though carriers may be healthy, these rearrangements can lead to an unbalanced set of chromosomes in offspring.
Embryos with unbalanced rearrangements often result in miscarriage, stillbirth, or a child with significant health issues. PGT-SR identifies embryos that have inherited these unbalanced configurations. Selecting embryos with balanced or normal structures reduces the risk of pregnancy complications and inherited chromosomal disorders.
Important Considerations
Preimplantation Genetic Testing provides valuable information about embryo health but is not without limitations. PGT is not 100% accurate; there is a small possibility of false positive or false negative results. It is a screening test, not a diagnostic one, and typically does not screen for all possible genetic conditions, such as polygenic disorders or certain de novo mutations.
Embryonic mosaicism is another consideration, where an embryo contains a mix of both genetically normal and abnormal cells. The biopsy samples are taken from the trophectoderm, cells that form the placenta, not the inner cell mass that develops into the fetus. This can sometimes lead to results that do not fully represent the entire embryo, as the abnormal cells might be confined to the trophectoderm.
While some mosaic embryos can lead to healthy live births, their transfer requires careful consideration. PGT also does not guarantee a successful pregnancy or a live birth, and further prenatal testing, such as amniocentesis, may still be recommended during pregnancy to confirm results.