Preimplantation Genetic Testing for Aneuploidy (PGTA) is a genetic test performed during in vitro fertilization (IVF). It screens embryos for chromosomal abnormalities before transfer to the uterus. PGTA aims to identify embryos with the correct number of chromosomes, improving the chances of a successful pregnancy. It offers insights into embryo health not visible through standard observation, helping select the most viable embryos for transfer.
Understanding PGTA
PGTA focuses on detecting aneuploidy, a condition where an embryo has an abnormal number of chromosomes. Humans typically have 23 pairs, totaling 46 chromosomes, in each cell. Aneuploidy occurs when there are too many or too few chromosomes, such as an extra copy (trisomy) or a missing copy (monosomy). This differs from Preimplantation Genetic Testing for Monogenic Disorders (PGT-M), which screens for specific single-gene conditions, or Preimplantation Genetic Testing for Structural Rearrangements (PGT-SR), which identifies structural changes in chromosomes.
Aneuploidy often leads to implantation failure, miscarriage, or the birth of a baby with a chromosomal condition. About half of all miscarriages are attributed to chromosomal abnormalities. Identifying euploid embryos, those with the correct 46 chromosomes, enhances pregnancy prospects.
Chromosomal Conditions PGTA Detects
PGTA identifies aneuploidies involving whole chromosomes, including extra and missing copies. One common condition detected is Trisomy 21, or Down syndrome, where an individual has three copies of chromosome 21. This extra genetic material affects brain and body development.
Another screened condition is Trisomy 18, or Edwards syndrome, characterized by an extra copy of chromosome 18. Individuals with Trisomy 18 often experience severe health problems, with many babies not surviving past their first year. Trisomy 13, or Patau syndrome, involves an extra copy of chromosome 13, leading to severe intellectual disability and multiple organ defects.
PGTA also screens for aneuploidies involving sex chromosomes. An example is Monosomy X, or Turner syndrome, which occurs in females with only one X chromosome. This condition can lead to short stature and other developmental issues. Other detectable sex chromosome aneuploidies include Klinefelter syndrome (XXY) and XYY syndrome.
Who Benefits from PGTA Testing
PGTA testing is often recommended for specific patient profiles in IVF. Women of advanced maternal age, typically over 35, are advised to consider PGTA. The risk of aneuploidy in eggs increases significantly with maternal age, with over 50% of embryos from patients over 40 potentially being aneuploid.
Individuals with a history of recurrent pregnancy loss may also benefit from PGTA. Chromosomal abnormalities are a common cause of miscarriage, and PGTA helps identify chromosomally normal embryos to reduce future losses. Patients who have experienced multiple IVF failures are another group for whom PGTA is considered.
Additionally, PGTA may be recommended for couples with a prior pregnancy or child affected by a chromosomal abnormality. This allows for screening embryos to reduce the chance of a similar condition in a subsequent pregnancy. PGTA aims to select the most viable embryos, potentially increasing the likelihood of a successful live birth.
The PGTA Testing Process
The PGTA testing process begins after eggs are retrieved, fertilized, and embryos are cultured. Embryos typically develop to the blastocyst stage, five to six days after fertilization, consisting of approximately 70-150 cells.
A small number of cells (3-10) are carefully removed from the trophectoderm, the outer layer of the blastocyst that will eventually form the placenta. This procedure, known as a trophectoderm biopsy, is performed with precision to avoid disturbing the inner cell mass, which develops into the fetus. The biopsied cells are then sent to a specialized genetic laboratory for analysis.
While awaiting genetic analysis results, embryos are typically cryopreserved, or frozen. Results are usually available within 7-10 days. Once results confirm euploid embryos, they are selected for transfer to the uterus in a subsequent cycle.