Definition & Overview
Pre-implantation genetic diagnosis (PGD) is a type of test performed before the implantation stage in an in vitro fertilization procedure. Its purpose is to screen the embryos for genetic diseases that they may inherit from their parents.
PGD combines the technological advancements in the field of genetics and assisted reproductive technology. It is performed after a successful in vitro fertilization procedure, which is done by using a laboratory dish to join the male sperm with the female egg. In the process of letting the sperm and the egg meet, several embryos may be produced. PGD is performed before transferring the embryos into the uterus.
PGD is different from pre-implantation genetic screening (PGS). While the former screens embryos for the existence of genetic problems, PGS tests the chromosomes to ensure the normalcy in their number.
Who Should Undergo & Expected Results
Pre-implantation genetic diagnosis is beneficial for people who have the possibility of passing on a genetic disease to their children. It is normally performed if one or both parents are aware of an existing problematic gene in their body or have an existing hereditary condition. The procedure is normally recommended for parents who meet the following conditions:
- If the parent had a previous pregnancy or pregnancies that ended as a result of a genetic condition
- If the parents have another child with a genetic condition
- If one or both parents are carriers of sex-linked genetic or single gene disorders
- If the woman is aged over 35 - This is because the chromosomal abnormalities and genetic diseases are more likely to occur in pregnancies of older women.
- If the woman has undergone more than one failed fertility treatments
- If the woman has had multiple miscarriages
Couples who choose to undergo the pre-implantation genetic diagnosis can expect the procedure to lessen their chances of conceiving a child with an inherited genetic disease. However, all parents who undergo the process should be made aware that PGD does not completely get rid of the possibility of conceiving a child with a genetic disorder. PGD is only the first of many tests that will be conducted repeatedly over the course of the mother’s pregnancy; together, these tests will determine the real genetic condition of the child.
PGD is also limited to detecting only a specific list of genetic conditions that are approved by the law. There are about 250 genetic diseases that are currently on the list, all of which meet a specific set of criteria. Generally, there are three major groups of diseases that PGD can diagnose; these groups classify the diseases as sex-linked disorders, single gene defects, and chromosomal disorders.
Examples of sex-linked disorders are hemophilia and some neuromuscular disorders; these are diseases that have something to do with the X chromosomes, which are inherited from the mother. On the other hand, diseases such as cystic fibrosis, muscular dystrophy, sickle cell anemia, Tay-Sachs disease, and Huntington disease are some examples of single gene defects. Lastly, chromosomal disorders result from an imbalance in the number of chromosomes, leading to chromosomal abnormalities in the child.
This special screening process also has the capability to screen embryos for adult onset diseases such as Alzheimer’s disease and other hereditary diseases such as breast cancer.
How Does the Procedure Work?
The procedure for pre-implantation genetic disease starts with in vitro fertilization (IVF), a process of fertilizing the egg outside a woman’s body. The procedure for IVF begins by suppressing the monthly menstrual cycle of the female partner using special medications. Once the disruption takes effect, the ovaries will be stimulated so that they will produce multiple eggs, which are then retrieved from the woman’s body by inserting a needle into the vagina and fertilized with the male partner’s sperm in a laboratory. The fertilization process typically takes two to three days. The embryo then divides into eight cells over the course of the next three days after the fertilization occurred; once this occurs, PGD can be performed.
The eight cells will then undergo screening, which is done by removing two cells first. If problematic genes are found, the embryo will be destroyed and another one will be tested. If no problematic genes are found, it will be placed into the uterus for the final implantation procedure. Unaffected embryos may also be kept frozen so that they could be used later.
Genetic disorders are tested using polymerase chain reaction (PCR), wherein the cells are placed into a tube and analyzed by DNA strand separation and replication to look for the presence of specific DNA sequences and gene markers pointing to a genetic disease.
Chromosomal abnormalities are detected using a process called array comparative genomic hybridization, which is done following a PCR. Array CGH, as it is known, checks the amount and number of chromosomes, and screens for known chromosomal rearrangements, also known as translocations. As with most procedures, an Array CGH comes with its limitations and may not always detect all types of chromosomal abnormalities.
Possible Complications and Risks
The risks involved with a pre-implantation genetic diagnosis are also the risks involved with in vitro fertilization. These include:
- Ovarian hyperstimulation syndrome
- Ectopic pregnancy
- Multiple births
- Reactions to fertility drugs, which may cause symptoms such as hot flushes, irritability, headache, and restlessness
The possible complications that can happen during a pre-implantation genetic diagnosis include the following:
- The embryo may be damaged during the process of cell removal.
- The result of the testing procedure may not be completely reliable. There is a chance of a genetic disorder being inherited despite the screening procedures performed.
- There is a risk that the in vitro fertilization process does not yield an embryo that is suitable to be transferred into the womb.
Failure of producing a suitable embryo may occur due to the following reasons:
- There are not enough eggs produced.
- There are not enough eggs fertilized.
- The embryos become damaged during the retrieval process.
All the embryos are carriers of a genetic disease.
Simpson JL, Holzgreve W, Driscoll DA. Genetic counseling and genetic screening. In: Gabbe SG, Niebyl JR, Simpson JL, eds. Obstetrics: Normal and Problem Pregnancies. 6th ed. Philadelphia, PA: Elsevier Churchill Livingstone; 2012:chap 10.