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Pre-implantation Genetic Test (PGT) comprises a group of genetic assays used to evaluate embryos before transfer to the uterus. The embryos can be tested for abnormal chromosomes before they are transferred to reduce the risk of abnormalities in the offspring.
The genetic defects include missing or an extra chromosome, single-gene disorder, or rearrangement of genes, which can cause pregnancy loss and birth defects.
PGT basically are of 2 types
Pre-implantation Genetic Screening (PGS)
Pre-implantation Genetic Diagnosis (PGD)
PGS is done when the genetic parents are presumed to be chromosomally normal and embryos are screened for aneuploidy.
PGS detects extra or missing chromosome/aneuploidy like down syndrome, Turner syndrome, hence it is called PGT-A
PGD is done when both/one genetic parent carries a gene mutation or a balanced chromosomal rearrangement
PGD is done to test whether the abnormality from a parent is transmitted to the embryo
When the embryo is tested for monogenic disorders, to detect inherited conditions and diseases like cystic fibrosis, sickle cell anaemia, hence it is called PGT-M and it is called PGT-SR when the embryo is tested for structural rearrangement such as translocation, inversion which can result in creating embryos with chromosomal issues.
For whom PGS is done?
In the case of advanced maternal age
History of recurrent early pregnancy loss
Severe male factor infertility
Repeated IVF failure
How is PGT done?
The first step is to fertilize the oocytes either by IVF or ICSI and then they fertilize and embryos are produced. the fertilized embryos are cultured till day 3 or day 5
The next step is to take a small biopsy of the embryo. the biopsy is performed either on day 3 by removing two to three cells or day 5 removing 3-6 cells.
Day 5 embryo consist of an inner cell mass(that develop into a fetus )and trophectoderm(that forms placenta), more cells are removed at this stage(removing little trophectoderm without compromising the viability of the embryo.
Once the cells are tested for abnormalities we can select the one that is genetically normal and transfer it into the uterus
What are the benefits?
Improves embryo selection
Reduce genetic transmission of unknown abnormalities