Preimplantation Genetic Diagnosis
Preimplantation genetic diagnosis, or PGD, is a diagnostic procedure that is used to detect genetic changes in embryos that are created using assisted reproductive techniques such as in-vitro fertilization. PGD can reduce the risk of having a child with a particular genetic or chromosomal disorder. In-vitro fertilization involves removing egg cells from a woman's ovaries and fertilizing them with sperm cells outside the body. To perform preimplantation testing, a small number of cells are taken from these embryos and tested for certain genetic changes. Only embryos without these genetic changes or abnormalities are implanted in the uterus to initiate a pregnancy.
These unique tests can evaluate the risk of several different conditions, including Down syndrome, Turner syndrome, Tay-Sachs disease, cystic fibrosis, muscular dystrophy and others, depending on the concerns and family histories of the couple. PGD provides genetic information at a much earlier stage than other prenatal diagnostic tests, such as amniocentesis or chorionic villus sampling, which may lead to the difficult decision of pregnancy termination if results are unfavorable.
Candidates for PGD
Preimplantation genetic diagnosis, or PGD, is a series of tests performed on embryos during the in vitro fertilization, or IVF, process. PGD allows couples who may be at risk for having a child with a serious genetic disorder, the ability to increase the chances of having a child without the disorder. Primary candidates for PGD may include:
- Patients with a family history of certain genetic disorders
- Patients over the age of 35
- Couples with a history of recurrent pregnancy loss
The PGD Procedure
The in vitro fertilization process is performed prior to a preimplantation genetic diagnosis. Testing is performed two to four days after fertilization has occurred and an embryo has developed. During the testing, one or two cells are extracted from each embryo to be evaluated for certain genetic conditions. PGD uses two basic testing methods:
- Chromosomal analysis to assess the number or structure of the chromosomes
- DNA analysis to detect specific gene mutations.
These tests are used to diagnose sex-linked disease, single-gene defects and chromosomal disorders. Sex-linked genetic disorders are passed from the mother to the child. They may include fragile X syndrome, hemophilia, neuromuscular dystrophies, Rett syndrome, pseudohyperparathyroidism and vitamin-D-resistant rickets. Single-gene defects include cystic fibrosis, Tay-Sachs disease, sickle cell anemia, and Huntington disease. Chromosomal disorders include aneuploidy, an abnormal number of chromosomes, which may result in repeated miscarriages. After testing is completed, only the healthy normal embryos are implanted into the uterus to promote a successful pregnancy.
Risks and Considerations for PGD
Risks associated with preimplantation genetic diagnosis include those associated with the IVF procedure and may include multiple births, ovarian cysts, and ovarian hyperstimulation syndrome. There is not enough research at this time to indicate whether this procedure hinders the implantation of the embryo into the uterus. Any potential long-term impact of PGD on the fetus and child has also not yet been studied.
PGD is a fairly new and developing technique and can be relatively expensive, which may discourage some patients already burdened with the cost of IVF. Since only one to two cells can be tested, inconclusive genetic test results are possible and can lead to misdiagnosis. As part of the IVF procedure, a test cannot be repeated and verified. Therefore, PGD is often confirmed later by other forms of prenatal diagnosis such as chorionic villi sampling or amniocentesis. A successful IVF and PGD procedure does not guarantee pregnancy after transfer, and a full term delivery is also not guaranteed.
Certain uses of preimplantation genetic diagnosis may be considered controversial, such as testing for late onset disorders such as Alzheimer disease, or for genetic susceptibility to diseases like hereditary breast cancer. PGD may also be performed to determine the gender of a child, although this purpose is considered extremely unethical and controversial.