Preimplantation Genetic Test, PGT
Chromosomal abnormalities in embryos increase rapidly with the mother's age. If there are chromosomal abnormalities in the embryo, the success rate of pregnancy decreases and the risk of miscarriage increases. Preimplantation Genetic Testing (PGT) is a procedure performed not only if the couple has specific chromosomal abnormalities but also when there is a high risk of chromosomal abnormalities in the embryo, which could lead to pregnancy failure or recurrent miscarriage.
In the past, to diagnose chromosomal abnormalities in embryos and implant only normal embryos, techniques such as Fluorescence In Situ Hybridization (FISH) were used to diagnose only specific chromosomes. However, recently, Next Generation Sequencing (NGS), which allows diagnosis of all 46 chromosomes, is more commonly used. Preimplantation Genetic Screening improves implantation rates and pregnancy rates during in vitro fertilization procedures, reduces miscarriage rates, and increases the likelihood of giving birth to a healthy baby.
Indications for Preimplantation Genetic Testing (PGT)
- When there is suspicion of numerical chromosomal abnormalities,
or if there has been a previous pregnancy with a chromosomal abnormality in the fetus
- Advanced maternal age
- Recurrent spontaneous miscarriage
- Repeated implantation failure
- Men with oligospermia
Numerical chromosomal abnormalities
(such as Down syndrome, Edwards syndrome, Patau syndrome, etc.)
Human chromosomes are expressed as 1 set (n) of 2 paired cells, with a total of 46 chromosomes, comprising 23 sets of chromosomes (2n=46). Numerical abnormalities of 2n chromosomes refer to deviations from the normal total of 46 chromosomes and are broadly classified into polyploidy and other categories.
- Polyploidy
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Abnormalities in chromosome sets, such as triploidy (3n=69) and tetraploidy (4n=92), are typical examples and are most commonly found in fetuses miscarried during early pregnancy.
- Aneuploidy
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Having one extra chromosome (trisomy, 2n+1=47) or one fewer chromosome (monosomy, 2n-1=45) is characteristic of aneuploidy. Most autosomal aneuploidies, like polyploidies, do not develop normally and result in either failure of pregnancy or miscarriage early in pregnancy.
However, some trisomies, such as Down syndrome (trisomy 21), Patau syndrome (trisomy 13), and Edwards syndrome (trisomy 18), can result in a viable pregnancy and birth, although they are associated with severe congenital abnormalities or intellectual disabilities. Except for Down syndrome, these conditions often lead to death shortly after birth.
Monosomies of autosomes typically do not result in a viable pregnancy and lead to early miscarriage. In contrast, most sex chromosome aneuploidies (such as Turner syndrome, 45,X; Klinefelter syndrome, 47,XXY; 47,XXX; 47,XYY) usually result in a viable pregnancy and birth. The symptoms are often mild or almost absent compared to autosomal aneuploidies, but these conditions can cause abnormalities in gamete formation, potentially leading to infertility or subfertility.
Safety of the fetus after Preimplantation Genetic Testing
The incidence of congenital abnormalities in babies born after preimplantation genetic testing is reported to be similar to that in babies conceived through standard in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI), and is comparable to the rate of congenital abnormalities in the general population.
Preimplantation chromosomal testing involves collecting only a few cells from the embryo for diagnosis, which may not fully represent the entire embryo. Due to the technical limitations of the testing method itself, it is important to consult thoroughly with your doctor before proceeding. If pregnancy occurs, an amniocentesis can be performed if necessary to confirm whether the fetus is normal.
Next Generation Sequencing (NGS) Schematic Diagram
The chromosome test before implantation is a method of collecting and Testing a small number of cells from the embryo. Due to the technical limitations of the test method itself, it should be conducted after consulting with the attending physician and if successfully pregnant, amniocentesis is required to confirm that the results are again within the normal limits.