PGT: preimplantation genetic testing
Preimplantation Genetic Testing (PGT)
Preimplantation genetic testing refers to the testing of chromosomes and genes in the fertilized egg stage prior to the transfer of an embryo obtained through in vitro fertilization.
PGT-A (Preimplantation genetic testing for aneuploidy) is a test for abnormalities in the number of chromosomes in a fertilized egg.
It is now known that most fertilized eggs contain 46 chromosomes (23 pairs of chromosomes in each cell) that contain genetic information, and that a low or high number of chromosomes results in failure to implant or miscarriage. PGT-A is expected to reduce the miscarriage rate and improve the success rate of pregnancies.
PGT-SR （Preimplantation Chromosome Structure Abnormality Test）
A portion of the cells of a fertilized egg obtained through in vitro fertilization are harvested to determine which embryos are free of chromosomal structural changes that would prevent miscarriage (or which embryos are balanced), thereby increasing the likelihood of a successful pregnancy.
Chromosome structural rearrangements are abnormalities in the size or arrangement of chromosomes, including genetic material, that have changed from their normal state. If one of the spouses is a carrier of a chromosome structural disorder, there is a high probability that an embryo with a chromosome structural problem will be produced, making it difficult to conceive and potentially resulting in a child with serious complications.
In carriers of chromosomal structural abnormalities, PGT-SR testing can help identify embryos that will increase the chances of establishing a healthy pregnancy; PGT-SR was formerly called preimplantation diagnosis (PGD).
Types of Preimplantation Diagnosis
PGT-A / Preimplantation Genetic Testing for aneuploidy
This test examines the number of chromosomes in an embryo. An embryo with an aneuploidy is called aneuploidy and is considered a cause of repeated ART failures and miscarriages.
PGT-SR / Preimplantation Genetic Testing for Structural Rearrangements
PGT-SR is a test for chromosomal structural rearrangements. This test is intended for patients with recurrent miscarriages and chromosomal structural abnormalities and is used to examine chromosomal structural abnormalities and prevent miscarriages after transplantation.
PGT-M / Preimplantation Genetic Testing for Monogenic
This is a test for genetic diseases. Couples who have the possibility of having a child with a specific genetic disease are eligible for this test, and the embryo is examined to determine whether it will not develop the disease and is implanted.
Preimplantation Genetic Testing (PGT)
PGT or Preimplantation Genetic Testing is a combination of Preimplantation Genetic Screening (PGT-A) and Preimplantation Genetic Diagnosis (PGT-M), where PGT-A tests for total chromosome number and abnormal placement, and PGT-M tests for specific genetic abnormalities in the female or male partner, such as cystic fibrosis or Tay-Sachs disease. Sacks disease, etc.) in the female or male partner. Genes or genetic information is usually arranged on chromosomes, and everyone has 23 sets of chromosomes: one set is produced by the egg and the other set by the sperm. After fertilization, the fertilized egg must have 23 sets of chromosomes arranged so that these two sets of chromosomes overlap exactly. With age, the structure within the egg becomes older, and the task of placing these chromosomes so that they overlap exactly becomes more difficult than when the egg is younger. For this reason, the number of fertilized eggs with an incorrect number of chromosomes increases with age. This fertilized egg with an incorrect number of chromosomes is called genetic aneuploidy (Aneuploidy). Down syndrome is the most well-known example of this genetic aneuploidy, in which one more chromosome 21 is present in addition to the 23 pairs of chromosomes, also known as 21 trisomy because there are three pairs of chromosomes. Preimplantation genetic screening (PGT-A) can reveal the chromosome arrangement and reduce the chance of implanting a fertilized egg with this abnormality. The accuracy of Preimplantation Genetic Screening (PGT-A) and Preimplantation Genetic Diagnosis (PGT-M) is approximately 99%. The chances of miscarriage can be greatly reduced by transferring a fertilized egg with normal chromosomes. Gender can also be determined. Preimplantation genetic testing (PGT) is usually performed five days after egg retrieval, or when the fertilized egg becomes a blastocyst. At this time, the fertilized egg already has many cells. The embryologist, using a microscope and laser, carefully punctures the shell of the fertilized egg and removes a few cells. These removed cells are placed in a special fluid and sent to a cytogenetics laboratory. This laboratory uses the latest and most advanced technology to test the fertilized eggs. The test results are sent to the clinic. The results are used to select which fertilized eggs will be transferred.
Preimplantation Genetic Diagnosis (PGD)
Preimplantation Genetic Diagnosis (PGD) is a wonderful option for people who are at risk of transmitting a genetic disease to their children. PGD allows the physician and embryologist to determine if an embryo carries a specific disease based on its genetic composition.
PGD testing is used to screen for abnormal numbers of chromosomes, broken or damaged chromosomes, and other anomalies, including
Down Syndrome (Trisomy 21)
Tay Sachs Disease
Hemophilia A and B
Sickle Cell Anemia
PGD performed on a woman’s embryos for the most common chromosomal problems enables the physician and couple to determine which embryos will most likely result in a healthy, ongoing pregnancy. Additionally, for couples that have failed IVF cycles for unexplained reasons, preimplantation genetic diagnosis may provide important information in the determination of the cause(s) of the failures.
PGD can also be used for gender selection.
PGD is conducted during an in vitro fertilization (IVF) cycle. The female undergoes ovulation induction with follicle stimulating hormone medications (FSH) to recruit numerous eggs. The mature eggs are withdrawn transvaginally and combined with the partner’s sperm as in a typical IVF cycle. The resulting embryos are cultured from 3-5 days during which time the PGD procedure is conducted. Only embryos without the genetic defect and with the correct number of chromosomes are transferred to the mother. PGD would be performed by using a laser. The laser affords embryos less exposure to handling and general outside conditions while undergoing PGD.
PGD Success Rates
Selective implantation of embryos with normal chromosome makeups has been shown to result in high pregnancy rates with decreased spontaneous miscarriage rates.
Gender Selection with IVF & PGS
Gender selection has become a popular option for couples desiring to choose the sex of their children. Preimplantation Genetic Screening (PGS) allows the patients to choose their baby’s gender by identifying male and female embryos conceived in a laboratory prior to transfer to the woman’s uterus. After fertilization, specialists examine the embryo for its sex chromosomes (XX for female; XY for male), and then implant an embryo of the selected gender into the woman’s uterus. As opposed to other gender selection methods for choosing the sex of the baby, PGD/PGS is 99.9% reliable.