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PGD and PGS: The Process
Preimplantation genetic diagnosis (PGD) and preimplantation genetic screening (PGS) are laboratory procedures that are performed on embryos created via in vitro fertilization (IVF). Therefore to create the embryos via IVF, a couple must go through the IVF process to create the embryos in order to get to the genetic screening process for embryos. The basic process for creating the embryos is as follows:
- The woman takes fertility drugs to stimulate ovarian follicle development and the maturation of multiple eggs.
- An egg retrieval is performed by a fertility doctor, usually in an outpatient surgical setting using a transvaginal ultrasound-guided aspiration procedure.
- The eggs are fertilized with the partner's sperm or donor sperm in a laboratory.
After the eggs are fertilized, the embryos are transferred to a special growth fluid and allowed to develop for several days. The single fertilized egg divides into two cells, then four and so on, a process of cell division, known as cleavage. Normally, by the third day after egg retrieval, healthy embryos will have developed into eight cells.
For PGD and PGS, a single cell is taken from a Day 3 embryo, or six to eight cells are taken from a Day 5 or 6 embryo, known as a blastocyst. A blastocyst is an embryo that has divided into hundreds of cells and has an inner cell mass and an outer cell mass, known as the trophectoderm.
On a Day 3 embryo, the embryologist to perform a biopsy with a special microscope and micromanipulators in order to remove an intact cell with minimal trauma to the remaining embryo. This is known as a cleavage-stage embryo biopsy.
For a blastocyst biopsy, cells are removed from the trophectoderm (outer layer of cells) using a fine biopsy pipette. The inner cell mass is left undisturbed. the embryo is then frozen to await results of the genetic testing.
The chromosomes examined for genetic abnormalities by geneticists and specialists using one of several techniques, including:
- Fluorescent in situ hybridization (FISH)
- Polymerase chain reaction (PCR)
- Single nucleotide polymorphism (SNP) analysis
- Comparative genomic hybridization (CGH)
In most cases, the genetic or aneuploidy testing can be completed within 24 hours of the embryo biopsy, allowing for a timely embryo transfer of the "good" embryos into the woman's uterus.