Pre-genetic diagnosis (PGD) is the use of genetic testing technology to assess and evaluate embryos for abnormalities. Also called preimplantation genetic diagnosis, PGD can be performed using many techniques available. In the in vitro fertilization (IVF) process, an egg is fertilized with sperm in the laboratory setting. After a few days, the resulting embryo is biopsied for genetic analysis. This allows the embryologist to evaluate if or not the embryos are genetically normal or if chromosomal abnormalities exist.
Why are embryos analyzed?
- Gender selection – For family balancing, the embryologist can assess the embryo’s gender.
- Aneuploidy – With this condition, there is an extra chromosome or a missing one. Down Syndrome is an example of aneuploidy, which is more common as a woman ages.
- Translocations – This refers to a change in chromosome structure, where chromosomes are attached to each other, or where pieces of two different chromosomes are interchanged.
- Recurrent pregnancy loss – Certain genetic defects can cause repeated pregnancy losses.
- Single gene disorders – Diseases or conditions caused by an inherited single mutated gene. Examples include Sickle Cell Anemia, Cystic Fibrosis, and Tay Sachs.
The PGD Process
The pre-genetic diagnosis can be done using different approaches. These are:
- Microarray method – This form of preimplantation genetic screening requires a blood test from the parents to create a DNA fingerprint. This can be used to compare the DNA from the assorted embryos to make sure there is one chromosome from each parent for every 23 pairs of chromosomes. This technique has the ability to test all chromosomes in less than two days. In addition, this test can be used to calculate the estimated accuracy of the result, which avoids errors.
- Fluorescent in situ hybridization (FISH) – With this method, a single cell is analyzed under the microscope. Fragments of DNA are specified to each chromosome being tested using microscopic markers and fluorescent single detection.
- Comparative genomic hybridization (CGH) – Also called a blastocyst biopsy, CGH tests the chromosomes expelled by the eggs during cells division after fertilization. This involves using cells of a day-5 embryo.
How accurate is PGD?
The accuracy of preimplantation genetic testing varies from couple to couple. There is a chance that PGD will not be 100% reliable. However, accuracy rates are 98% for most couples.
Once PGD testing results are available, the fertility specialist discusses them with the clients, and only normal embryos will be used for IVF cycles. One or two embryos are placed in the woman’s uterus, and extra embryos are frozen for later cycles. PGD allows the doctor to use genetically normal embryos for transfer, increasing the chances of IVF success. When all chromosomes are evaluated, and assessed as normal, the chances of pregnancy is around 85-90%.
PGD for Failed IVF Treatment
When a couple experiences a failed IVF cycle, PGD is recommended. One of the most common reasons IVF fails is genetic abnormalities of the embryos. Women may have factors that cause their ovaries to produce genetically abnormal eggs, which produce problems with the embryos. This will lead to unsuccessful IVF treatment. Egg donors can be an option when this is found. If the male’s sperm contributes the abnormal genetic material, donor sperm may be used.
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