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Preimplantation Genetic Testing (PGT)

by in Blog 10/24/2021

What is PGT?

PGT is the DNA analysis of the oocyte (polar body) or of the embryo (in cleavage or blastocyst stage) in order to identify the HLA type (human leukocyte antigens) or genetic anomalies.

All preimplantation genetic testing techniques require a procedure of in vitro fertilisation.

There are 3 types of PGT, each with certain specific indications.

What are the preliminary stages leading to PGT?

Before PGT, female patients must undergo an in vitro fertilisation procedure in order to obtain a sufficient number of day-5 embryos in blastocyst stage.
The fertilisation technique and the stimulation treatment are chosen depending on the specific details of each infertility case. The female patients go through all the steps necessary for the complete diagnosis of their infertility, followed by controlled ovarian stimulation and ultrasound-guided ovarian drilling in order to obtain embryos in blastocyst stage.
Biopsy techniques
A single-use pipette is used to collect fresh or vitrified embryos which are then processed in carefully controlled environments designed specifically for such purposes.
Biopsy technique:
The incision of area pellucida on days 3-4 of culture, and the excision of the trophectoderm cells on days 5-7 of culture.
The early incision of the area pellucida on the day when the blastocyst is formed and then continuing with the culture until the herniation of trophectoderm cells, followed by their excision.
The incision of the area pellucida and the simultaneous excision of the trophectoderm cells of the fully expanded blastocyst.
After the biopsy, the embryos are cryopreserved until the results become available.
Then, only the euploid embryos are transferred. In case the couple is only able to produce mosaic embryos, these may be transferred with the prior consent of the couple and the clinician.

How are the biopsied cells tested?

The test used for visualising genetic material and detecting anomalies is the NGS test. It entails the scanning of the entire genome in search of chromosomal, genic, or structural anomalies.

How long does it take for the results to come in?

This procedure requires the testing of a large number of embryos using special kits made in the USA. This is why the results may take several months to a year to become available, depending on the number of embryos which need to be assessed for preimplantation.

Is it possible to not have any euploid embryos?

The results of preimplantation genetic diagnosis can reveal the following situations:
The embryos are euploid, in which case they may be transferred
The embryos are mosaic (some cells feature genetic mutations), in which case they could be transferred if euploid embryos are absent
The embryos are aneuploid and cannot be transferred
The results are inconclusive, in which case a new biopsy may be performed.

dr Ana Maria Crețu, medic rezident Obstetrică-Ginecologie