Preimplantation Genetic Diagnosis (PGD)
Preimplantation genetic diagnosis (PGD) allows for the detection of life-altering genetic abnormalities prior to embryo transfer during IVF. The procedure can also help patients suffering from certain fertility challenges as well.
PGD is recommended for patients who are at an increased risk for genetic disorders. These are couples who are carriers for diseases such as Cystic Fibrosis, Tay Sachs, Sickle Cell, Huntington’s chorea, or Thalasemia. It is also helpful for patients who have no other known reason for infertility (unexplained infertility), recurrent pregnancy losses, advanced maternal age, repetitive IVF failures, or male factor infertility.
The role of chromosomes in reproduction
The PGD procedure is an evaluation of chromosomes, thread-like structures found in all human cells except red blood cells. Chromosomes, which contain human genetic material (DNA), come in pairs, and a normal human cell contains 46 chromosomes, 22 pairs of autosomes and two sex chromosomes.
Chromosome abnormalities include aneuploidy, an abnormal number of chromosomes, and structural abnormalities, which include translocations, inversions, and deletions of certain chromosomes. Both kinds of chromosomal abnormalities can be present in the egg or sperm, and the most common abnormality is aneuploidy.
The transmission of a chromosome abnormality to an embryo can result in a miscarriage, pregnancy complications, low implantation rate, or the birth of a baby with a genetic disorder.
As a women gets older, the chromosomes in the egg do not divide normally; therefore, it is possible that too many chromosomes are present in the embryo, which could cause birth defects, pregnancy loss, and/or mental retardation. For example, Down’s syndrome is due to an extra chromosome 21.
Women over the age of 35 fall into the advanced maternal age category. Approximately, 62% of embryos from women of advanced maternal age may have an abnormal number of chromosomes, or aneuploidy.
Consequently, they are at a higher risk of implantation failure, miscarriage, or the birth of a child with a chromosome abnormality such as Down syndrome. But with PGD, a geneticist can know whether the embryo is chromosomally normal within 24 hours, thus saving the parents undo stress for nine months.
The PGD process
The PGD process starts after the embryo is fertilized and allowed to grow, when an embryo biopsy is performed. This procedure involves removing either a polar body, which is half of the maternal DNA, or a blastomere, which is a cell from the embryo that contains its DNA.
Either the polar body or the single blastomere is subjected to FISH. This technique inserts several probes which recognize specific chromosomes and “hooks” onto its genetic target it when it is located. These probes glow a single color which allows detection.
With FISH, technicians can identify the absence or presence of specific genetic disorders in each developing embryo. As a result, a fertility doctor would only transfer those embryos free of genetic diseases as to increase the chance of conception and ultimately a healthy baby.