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PRE-IMPLANTATION GENETIC TESTING (PGT)
WHAT IS PRE IMPLANTATION GENETIC TESTING (PGT)?
Previously used terms like PGD (Pre Implantation Genetic Diagnosis) and PGS (Pre Implantation Genetic Screening) has been replaced by a more appropriate term called PGT (Pre Implantation Genetic TESTing). PGT is defined as a test that is performed on few cells that are extracted (embryo biopsy) from the Embryo (mostly Day 5 Blastocyst) for ruling out genetic abnormalities.
HOW IS PGT DONE?
PGT is done in conjunction with IVF Cycle, wherein the embryo’s formed during the cycle, are biopsied to extract a few cells and those cells are sent to a genetic lab for testing their genetic competency.
The following diagram illustrates the entire process followed during an IVF – PGT Cycle:
As shown in the illustration, up until Step 4 of Embryo Biopsy, the events in the IVF Cycle are similar to a regular IVF Cycle i.e
Step 1 : Ovarian Stimulation (with Follicular Monitoring)
Step 2: Oocyte Retreival or Ovum Pick-Up (And later Fertilization in the IVF lab)
Step 3: Embryo Culture (Upto Blastocyst Stage – Day 5 to 6 Embryo)
Step 4 : Embryo Biopsy (Extracting few cells from Embryo)
Embryo Biopsy is usually performed on Day 5 or Day 6 when we have fully expanded Blastocyst. During Embryo Biopsy, a few cells (about 7 to 10 cells) are extracted from the Blastocyst using Biopsy needle and the extracted cells are then safely transported to a Genetic Lab for testing.
Step 5 (Genetic Lab): Pre-Implantation Genetic Testing
The extracted cells are tested using novel NGS – Next Generation Sequencing technique which is a kind of molecular genetic testing which analyses all 23 pairs of chromosomes in details.
PGT report usually takes about 1 to 3 weeks depending upon the type of testing requested (PGT-A, PGT-SR or PGT-M). Results of PGT indicate which embryo is normal and which ones are abnormal.
Step 5 (IVF Lab): Embryo Cryopreservation
Meanwhile in the IVF Lab, the Blastocysts that underwent biopsy on Day 5/6 are then tagged and cryopreserved for later use.
Step 6 (Arrival of PGT Result): Results indicate genetic competency of Embryo’s. The ones that have multiple abnormalities are discarded.
Step 7 and Step 8: FET Cycle
Patients are then prepared for Frozen Embryo Transfer Cycle by giving medication to prepare the endometrium. Based on the report of PGT, a normal embryo with potential to implant is transferred to the patient.
HOW MUCH TIME DOES IT TAKE TO GET PGT RESULTS?
Embryo Biopsy result from a genetic lab usually takes one to three weeks depending on the type of test requested.
PGT-A and PGT-SR usually takes about 7 to 10 days.
PGT-M takes about a maximum of 3 weeks. During PGT-M patients can also be requested to get their blood Karyotyping test done before the Embryo biopsy.
In Blastocyst stage Biopsy, a few cells are extracted from the outermost layer of the Blastocyst (Trophectoderm). This approach is considered to be the safest approach for biopsy without impacting the implantation potential of the embryo to a great extent.