Preimplantation Generic Testing
In procedures such as in vitro fertilization (IVF), embryos are created in a laboratory and then transferred to the woman’s uterus. For some couples, especially those at risk for genetic disease, questions arise about the chromosomal make-up of those embryos. A technique called preimplantation genetic diagnosis (PGD) can provide information about some of those abnormalities.
What Is PGD?
There are two types of PGD. The first technique screens couples who have a known risk for inherited diseases such as cystic fibrosis, Tay-Sachs, muscular dystrophy, and chromosomal abnormalities, among others. The second screens for aneuploidy to test for common chromosomal disorders associated with advanced maternal age or recurrent pregnancy loss.
PGD is a highly sophisticated procedure that is offered by a limited number of facilities. Some centres limit its availability to couples who have experienced repeated miscarriages due to genetic disorders, or who already have had one pregnancy/child with a genetic issue and face the same high risk in subsequent pregnancies. Some centres make the procedure available to women of advanced reproductive age.
Since PGD is time sensitive, only available in a few clinics and the process requires advanced preparation, couples considering PGD should make arrangements well in advance of their IVF cycle.
What Can PGD Detect?
PGD can identify specific genetic abnormalities listed below or chromosomal abnormalities such as Down syndrome. The gender of an embryo can be determined with PGD. Therefore, couples at risk of having children with gender-linked diseases, such as the male-specific hemophilia, X-linked mental retardation, or muscular dystrophy, can use PGD to identify, then transfer, only embryos that are not at-risk.
As PGD procedures are refined, the number of disorders that can be diagnosed continues to increase, and the potential for diagnostic errors decreases. Couples should contact their IVF centres about whether PGD is available for a specific condition.
PGD can detect disorders such as cystic fibrosis, Charcot-Marie-Tooth disease, Down syndrome, Duchenne muscular dystrophy, Fanconi's anemia, Huntington's disease, Marfan syndrome, muscular dystrophy, sickle cell anemia, thalassemia, spinal muscular atrophy type I and Tay-Sachs disease.
The PGD Procedure
During PGD a cell is removed from an embryo when it has grown to eight cells, usually three days after fertilization. That cell is analyzed for certain genetic conditions using one of two basic techniques: fluorescent in situ hybridization (FISH) used to detect selected chromosome numbers, or polymerase chain reaction (PCR) based DNA amplification used to detect gene mutations.
Even though the risk of actually having a pregnancy with a chromosomal abnormality is low, not all embryos produced will have the entire chromosomal make-up needed to result in a healthy baby. Most of those embryos will not develop or implant. This is why the natural pregnancy rate even in fertile women may only be 20 to 30% a month. As a woman gets older the likelihood of a having a child born with a chromosome abnormality increases, from 0.3% at age 33 to 5% at age 45.
PGD aneuploidy screening can help identify those embryos that have tested normal for the most common chromosomal abnormalities.
The American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology states that, worldwide, at least 1,000 babies have been born following PGD. There have been no reports of increased malformation rates in liveborn infants previously analyzed by PGD, compared to rates in spontaneously conceived infants. Long-term studies have been difficult to conduct due to loss of contact with children previously subjected to PGD, and the absence of a unified classification system among centers regarding major and minor malformations. Such results may become available with the organization of future follow-up studies.
An endometrial biopsy can also evaluate abnormal uterine bleeding, which is also generally reflective of hormone imbalances.
How It Is Done
This procedure involves removing tissue from the uterine lining for laboratory analysis. This may be done in the physician's office and usually doesn't require anaesthesia.
The procedure is usually done three to seven days before a woman’s menstrual period is expected to begin. Before an endometrial biopsy, it is important to make certain that the woman is not pregnant.
The procedure involves inserting a catheter (a flexible tube) through the cervix and into the uterus, where the healthcare provider can then remove a piece of tissue from the uterine lining. The procedure itself may cause mild pain or cramping. Some physicians may recommend taking ibuprofen prior to the procedure to help minimize the discomfort.
The tissue sample is then sent to a laboratory for evaluation of the cell types present at that particular time of the menstrual cycle. The types of cells present in the uterine lining change throughout the monthly cycle, and a diagnosis can be made based upon the cell type seen and the day of the menstrual cycle on which the sample was taken.
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