Preimplantation Genetic Diagnosis
Preimplantation Genetic Diagnosis (PGD) is valuable to determine possible genetic disease.
Genetic carrier screening is done to determine if one or both parents may have abnormal genes that may increase the chance that their child will have a specific genetic disease. For many genetic diseases, if someone has an abnormal gene, that person is considered a carrier for that genetic disease. If this abnormal gene is passed to the child, the child will usually not be affected with that genetic disease but will be a carrier for that genetic disease. If both parents are carriers of the abnormal gene for the same genetic disease, there is a 25% chance that their child will inherit one abnormal gene from each parent and be affected with that genetic disease. Genetic screening is typically done on one parent first, and if the first parent tests positive, then the other parent is tested. The American College of Obstetricians and Gynecologists (ACOG) recommends screening for certain genetic diseases when indicated due to ethnicity, family history, or other known risk factors.
Cystic Fibrosis (CF) and Spinal Muscular Atrophy (SMA) are genetic diseases that may be screened for. It is estimated that 1 in 31 Americans are carriers of a mutation of the CF gene and 1 in 40 are carriers of a mutation in the SMA gene. Because so many people are carriers of the mutations for CF and SMA, across all races and ethnic groups, ACOG currently recommends that routine screening for Cystic Fibrosis be offered to all couples who are planning to have a baby and the American College of Medical Genetics recommends that all couples be offered carrier testing for Spinal Muscular Atrophy.
ACOG currently recommends that carrier tests for the following genetic diseases be offered to all individuals of Ashkenazi Jewish descent who are having a baby, because of the high carrier frequency rate in this ethnic group: Gaucher’s Disease (1 in 15), Tay Sachs Disease (1 in 30), Familial Dysautonomia (1 in 30) and Canavan Disease (1 in 40). ACOG currently recommends that carrier tests be offered only to individuals who have a family history of the following very rare genetic diseases due to the low carrier frequency rate: Fanconi Anemia Group C (1 in 89), Niemann-Pick Disease Type A (1 in 90), Bloom Syndrome (1 in 100) and Mucolipidosis type IV (1 in 127).
While Tay Sachs Disease is less common in Caucasians who are not of Ashkenazi Jewish descent, it has been observed to be more common in individuals of French Canadian or Cajun descent. Consequently, screening for Tay Sachs is also offered to these individuals.
There are other genetic diseases that are transmitted directly from parent to child. If the parent is determined to have the gene causing that genetic disease, there is a 50% risk of the child being affected by the same genetic disease. Certain genetic diseases that are carried on the sex-determining (X & Y) chromosomes may cause disease primarily in male children but only rarely in female children. ACOG currently recommends that screening for Fragile X Syndrome be offered only to women with a family history of mental retardation, developmental delay of uncertain cause, autism or autistic-like behavior, as well as women with premature ovarian failure or elevated follicle stimulating hormone level before the age of 40 years, but not to the general population.
During the course of your evaluation and treatment at IVF New England, your physician may recommend screening for specific genetic disease(s) which may be indicated based on your medical history and/or family history, to determine whether or not you are a carrier for the specified genetic disease(s). There is no one genetic carrier test that detects all genetic diseases and therefore genetic carrier tests may only be done for specified genetic diseases and are usually performed on a blood sample.
Genetic testing is a valuable tool but there are limitations:
Negative result – The genetic testing laboratory usually tests for the most common mutations (change in gene structure) and may not identify the less common mutations. So it is possible to have a negative test result but still have a genetic mutation that was not or could not be identified by the testing laboratory due to limitations of current technology.
Positive result - A positive test result indicates that you are a carrier for a genetic mutation that can cause a specific genetic disease or can put you and/or your child at risk for developing a disease. If you are determined to be a carrier, your reproductive partner will then be advised to undergo genetic carrier testing.
Inconclusive result – Sometimes it is not possible for the testing laboratory to determine genetic mutations. In this case, the genetic carrier test may need to be performed again at the same or different testing laboratory.
This information is provided by IVF New England for general education purposes and is not intended to take the place of a discussion with your physician. If you have questions about any aspect of your health, you are advised to speak with your physician.