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Pre-implantation Genetic Testing

( Pre-implantation Genetic Testing, PGT )

Pre-implantation Genetic Testing (PGT) is to screen the chromosome or genetic materials of the embryo before transferring it to the woman’s uterus to determine the genetic disorder. PGT is classified into Preimplantation Genetic Testing for Aneuploidy (PGT-A) and Preimplantation Genetic Testing for Mutation (PGT-M).

PGT is possible for the following cases:

1. A man or woman or both of them have a background of genetic disease that might be transferred to the linear, which has a risk of non-survival or abnormality.
2. A man or woman who has no genetic disorder might take PGT in the following cases:
   1. Have a history of pregnancy that the baby has a severe genetic disorder that might be prevented by PGT.
   2. Have a child with the disease that can be treated with stem cell transplantation from another child with HLA matching.
      Embryo’s HLA testing before transferring is useful. With embryo pregnancy, stem cells from cordocentesis can treat the ill child with HLA matching.
   3. Have a history of 2-3 miscarriages or more before 12 weeks, or the examination result demonstrates that the cause of previous carriage is the genetic disorder in a baby.
   4. A woman aged 35 years old or over with a medical indication that the embryo has a risk of a genetic disorder.
   5. No pregnancy twice consecutively after having assisted reproductive technology.

Steps of PGT

The first step is as same as the IVF, starting from the ovarian stimulation to the fertilization preparation and growing the embryo to the appropriate phase for PGT, which is divided into three stages.

1. Oocyte before fertilization or the pre-embryonic stage. Polar body of oocyte or pre-embryonic stage.
2. The embryo divides on Day 3 after fertilization (Clevage Stage).
3. Embryo divides at Day 5 (Blastocyst Stage)

In general, the doctor examines the embryos at the Blastocyst Stage because there are enough cells for examination with the fewest impact on the embryo growth and the difference of genetic materials in each cell might be detected, which is called Mosaicism. When the embryo reaches the cell division stage and has enough cells in the placenta, which is regularly 3-5 days after fertilization, the cells of each embryo are sucked under high magnification for genetic testing to identify if the embryos are normal or not. Select the strongest embryo with no disease to transfer back to the uterus for implantation and pregnancy. Currently, the data of an infant born through this technique is at a high rate, and their development is normal. For this reason, it is acceptable that the infant born through this technique has no difference from the infant born through a natural pregnancy. Therefore, the testing is the genetic test of the screening cells, and the obtained cells are the representatives of the entire cells of the embryo since the testing is more deliberate and complicated than testing with blood or other tissues because the genetic materials in one cell are low. As a result, the testing requires accuracy, time, and high experience from doctors or scientists for the highest precision and less disturbance to embryos.

PGT Technique

Today, PGT has various techniques depending on the genetic disorder, single gene disorder, or numerical aberration. Each technique has different complications and diagnosis precision.

1. Fluorescence Insitu hybridization (FISH) is a process that uses fluorescent probes to detect the target chromosome to determine the number of chromosome pairs, which are the 23rd, 18th, X, and Y chromosomes. This technique requires a lower cost but gives a level of precision. However, it does not examine the genetic materials of all embryos, so it is not frequently used for embryo screening.
2. Comparative Genomic Hybridization (CGH) is the PGT technique by analyzing 23 pairs of chromosomes with the Whole Genome Amplification (WGA) technique and testing for the target genetic materials by labeling the genetic materials on embryo and standard genetic material to compare with the different fluorescent colors. Interpret the result with a computer program after the reaction. Results identify the Aneuploidy, Unbalanced Translocation, or Chromosome Deletion or Duplication. Nevertheless, this technique cannot analyze the abnormality of the balanced translocation or inversion because such abnormalities do not differentiate the number of genetic materials from the standard one. The technique takes about 24-72 hours.
3. Next Generation Sequencing (NGS) is a currently popular technique. It is the technique to analyze the type and sequencing of embryo genetic material that detects the chromosome abnormality to the base sequence, which is the smallest component of the chromosome. Therefore, it has high precision. The strength of this technique is the abnormality detection of all 24 chromosomes with 99.9% precision within a short time. However, it cannot detect the diseases caused by the single gene disorder, mosaicism, polyploidy, and aneuploidy of the genetic materials part in the small chromosome that the technique can detect, such as aneuploidy of the chromosomes that are smaller than 10 megabase.

For treatment guidelines for a couple at Genesis Fertility Studio, a team of doctors gives the treatment guidelines and technique by individualization based on the current medical indication for the most appropriate and precise outcome at the appropriate time and cost to suit each couple. We ensure that the best technique and approaches are offered to you, along with the novel certified techniques.

Limitations to Preimplantation Genetic Diagnosis

PGT has a limitation to the precision of results; the precision is 95-99%. However, this technique cannot replace the prenatal diagnosis. Thus, even though the PGT results show normality, it is recommended to take prenatal diagnosis as same as the natural pregnancy. Besides, a couple should acknowledge the possibility of having the appropriate embryo for the transfer because the cause of infertility and the quality of gamete of each couple is different. Preimplantation genetic diagnosis might have a low number of normal embryos or different numbers of embryos in each cycle, especially in a person who has a screening for the disease caused by a single gene disorder, such as thalassemia, and HLA that matches with the child with the disease. Therefore, a couple might have to have ovarian stimulation to get the appropriate and sufficient embryos for disease screening and pregnancy.