Preimplantation Genetic Testing
Pre-implantation genetic screening/ Pre-implantation genetic diagnosis are genetic testing procedure done on an early embryo prior to embryo transfer. This procedure is offered to couples with a genetic abnormality or those who are at the risk of passing genetic defects to the child. PGS/PGD helps to reduce pregnancy losses and prevent the undesired genetic traits.
Pre-implantation genetic testing is done on individualised basis. Before taking a decision, each couples history and other fertility-related factors are analyzed and then this option is offered to them.
1) Pre-implantation genetic screening (PGS):
PGS involves screening the embryos for chromosomal abnormality, to make sure they have right number of chromosomes (23 pairs of chromosomes) without any extra or missing one (addition or deletion of chromosomes).
PGS is usually offered in:
- Women over the age of 39, undergoing IVF.
- Couples with recurrent miscarriages (two or more genetic or unexplained losses)
- IVF Failures (two or more failures, despite normal-appearing embryo)
- A positive history of chromosomal abnormalities in the family or child born with a chromosomal abnormality.
2) Pre-implantation genetic diagnosis (PGD):
PGD involves testing of embryos for single gene defect or inherited mutations which have already been documented in one or both the parents. The embryos which do not carry genetic disorders are selected for transfer.
PGD is recommended in:
- Carrier of single-gene disorders like Cystic Fibrosis, Sickle cell disease, Thalassemia, Hemophilia
- One or both partners or family history of heritable genetic disorders
- One or both partners is a carrier of chromosomal abnormality
- Couples who have children/children affected by a single gene disorder.
- Both PGS & PGD involves testing of cells from embryos that are obtained via IVF/ICSI. Embryo biopsy can be done at different stages of embryonic development.
- Blastomere Biopsy– usually done on Day three cleavage stage embryos. One or two cells from 8 cell embryo are removed and sent for genetic testing.
- Blastocyst Biopsy– usually done on Day five Blastocyst stage embryo. Four to five cells are removed from the outer trophectoderm layer without affecting the inner cell mass which develops later into a foetus. Blastocyst biopsy is generally preferred as many cells biopsied allows accurate genetic assessment.
Once the embryo biopsy is completed, the embryos are frozen until the tests result to arrive. Upon the availability of the test results, the genetically normal embryos are thawed and transferred into the womb.