Genetic Screening for Birth Defects
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Genetic Screening for Birth Defects
Birth defects, which occur in nearly one in 20 pregnancies, range in severity from minor anatomic abnormalities to extensive genetic disorders or mental retardation. Some couples have a greater than average risk of having a child with a birth defect. Genetic screening refers to the use of specific tests to determine which members of a population are at increased risk for an inherited condition. Genetic testing, in contrast, is the use of specific tests to characterize the genetic status of an individual who is suspected to be at increased risk for an inherited disease. These terms are frequently used interchangeably. Genetic screening may help identify couples who have an increased risk of age-related or familial genetic disorders and birth defects. No single test, however, can accurately predict the risk of all defects in a child, and many birth defects, such as those related to environmental and toxic exposures and those that are random and unexplained, are not genetically based and may not be detected with genetic screening.
Screening for genetic diseases that may affect offspring depends upon the racial or ethnic background of the couple, their family and medical history, and associated conditions. Various racial and ethnic groups demonstrate an increased prevalence of specific diseases, and couples of these backgrounds may have their carrier status screened accordingly, as described below. Most couples have their carrier status checked if there is a family history of the disorder or if they belong to an at-risk racial or ethnic group.
In addition to screening for carrier status for genetic conditions, pregnant women may also undergo prenatal testing to determine if their child is affected by a serious chromosomal abnormality, such as Down syndrome, or an abnormality of the spinal cord such as spinal bifida. Any woman may elect to have her risk for these disorders characterized by noninvasive testing, such as measurement of alpha-fetoprotein in the mother's blood as a marker for risk of spina bifida and other neural tube defects. If the screening test identifies that she is at increased risk, specific diagnostic tests may be performed, such as ultrasound examinations and amniocentesis for spinal bifida. One of the most common reasons to have genetic screening performed is the mother’s age, since the risk of having a child with a chromosomal abnormality increases as a woman ages. Additionally, if both parents are carriers for a genetic condition, prenatal testing can determine if the unborn child is affected with the disease.
Preimplantation genetic diagnosis (PGD) is a technique used in conjunction with in vitro fertilization (IVF) to test embryos for specific genetic disorders prior to their transfer to the uterus. PGD makes it possible for couples or individuals who have or who carry serious inherited disorders to decrease the risk of passing the disorder on to their child. This technique is controversial and raises issues of sex selection and genetic engineering. At present, PGD is offered only in a few centers, and may be offered under the supervision of an institutional ethics review board, but its use is increasing.
Indications for Genetic Screening-Advanced Maternal Age
Women over 35 years of age, those with a previous affected child, and those with a positive family history have a higher risk of chromosomal problems and miscarriage. Prior to attempting pregnancy, women in this age group and those with high risk histories may wish to consult their physician or a genetic counselor about their chances of having a child with a chromosomal problem, such as Down syndrome, and the choice for prenatal genetic testing if pregnancy is achieved. Non-invasive screening may be performed during the first trimester or second trimester of pregnancy by combining sonographic evaluation of the fetus with biochemical testing of the mother's blood. A normal screening test means that the risk of a chromosomal abnormality is reduced, but not zero. Chorionic villus sampling and amniocentesis are two methods of confirming the diagnosis of a chromosomal abnormality when the screening test is positive. Both of these tests carry a small risk of miscarriage. Many parents want to know this information so they can make informed decisions about their pregnancy.
Racial or Ethnic Associations to Specific Diseases
- Sickle Cell Disease: Anyone with African-American ancestry should be screened via hemoglobin electrophoresis for carrier status of this disease, as one in 10 may be a carrier.
- Cystic fibrosis (CF): It is estimated that 3% to 10% of Caucasians carry a defective CF gene, but do not have symptoms because a person must inherit two defective CF genes, one from each parent, to develop the disease. CF is the most common, serious, inherited disease in Caucasians, and is more common in those of northern or central European background and Ashkenazi Jewish background.
- Thalassemia: People of Greek, Italian, Mediterranean or southern Asian descent experience a high incidence of this disease. Patients can have a complete blood count (CBC) with mean corpuscular volume (MCV) to rule out the possibility of thalassemia. An MCV of < 80 should be evaluated further by hemoglobin electrophoresis. About 3% of the world’s population carries a gene for thalassemia.
- Tay Sachs: This disease has a high incidence in Eastern European Jews and French Canadians.
Familial Associations with Specific Diseases
A family history of any of the following disorders should prompt genetic counseling, and when relevant, screening or testing for associated gene abnormalities or carrier status:
Medical History/Conditions Associated with Genetic Conditions
- Recurrent miscarriages may be due to chromosomal or genetic abnormalities. Women who experience two or more consecutive miscarriages are frequently screened to see if they carry abnormal chromosomes that predispose to miscarriage.
- Congenital bilateral absence of vas deferens (absence of the two muscular tubes that carry sperm from the epididymis to the urethra, CBAVD) is associated with CF. The majority of men with CBAVD have CF. For couples where the man has CBAVD, the CF carrier status of the female partner must be determined.
- Azoospermia (absence of sperm in semen) and severe oligospermia (very low sperm counts) have been associated with chromosomal and genetic abnormalities. Blood tests, including a karyotype (which looks at the number and structure of the chromosomes) and Y chromosome micro-deletion analysis (which looks for missing pieces of the Y chromosome), may be helpful in selected cases. If abnormalities are identified, genetic counseling is indicated.