• Types of Tests
• Ethical Issues

Types of Tests  (back to top)

In general, reproductive genetic testing involves the following categories of tests:

Carrier screening is performed to determine whether an individual carries one copy of an altered gene for a particular recessive disease. The term recessive refers to diseases that will occur only if both copies of a gene have a disease-associated mutation; thus, each child born to two carriers of a gene mutation has a 25 percent risk of inheriting two altered genes, one from each parent, and thus being affected with the disorder. Examples of carrier tests include those for Tay-Sachs disease, sickle cell anemia, and cystic fibrosis. Couples are likely to have carrier tests if they are at higher risk of having a child with a specific disorder because of their family medical history or racial or ethnic heritage.

Preimplantation genetic diagnosis (PGD) is used following in vitro fertilization to diagnose a genetic disease or condition in an embryo. Preimplantation genetic diagnosis is essentially an alternative to prenatal diagnosis, as it allows testing before a pregnancy begins. Doctors can test the polar body of an egg cell or cells from an eight-cell embryo to identify embryos free from genetic disease or with specific traits. These embryos are transferred to the woman's womb to initiate a pregnancy. Over 1000 babies have been born worldwide after undergoing the procedure, and the number is growing rapidly.

Preimplantation genetic diagnosis has been applied to patients carrying chromosomal rearrangements, such as translocations, or those at risk of transmitting a single gene disorder to their offspring. Preimplantation genetic diagnosis techniques have also been used to detect chromosomal abnormalities in women of advanced maternal age undergoing fertility treatment. These patients usually do not have a known heritable mutation or chromosomal abnormality. Rather, PGD is used to detect chromosomal abnormalities arising in meiosis or early rounds of mitosis that are more common in older women. More than 100 different single gene disorders have been diagnosed in preimplantation embryos and the number is increasing each year.

Prenatal testing is used to diagnose a genetic disease or condition in a developing fetus. Two broad categories of tests are available, invasive procedures in which a sample of the fetus' tissue is obtained, or non-invasive procedures in which the fetus is visualized or a sample of mother's blood obtained for testing.

Amniocentesis and chorionic villus sampling (CVS) are the two most common invasive tests performed. Amniocentesis is done at 14 to 20 weeks gestation and involves inserting a needle through the mother's abdomen into the amniotic sac to obtain a sample of the amniotic fluid. The fluid can be tested for biochemical disorders and the fetal cells in the fluid can be tested for chromosome or gene abnormalities. CVS is done at 10 to 12 weeks gestation and involves inserting a catheter through the mother's cervix or abdomen to obtain a sample of the placental tissue that can be analyzed for chromosome or gene abnormalities. Invasive tests involve some risk to the fetus.

The most common non-invasive tests are ultrasound and maternal serum screening. Ultrasound can be done at any time during pregnancy to visualize the fetus, to determine the size and position of the fetus, the size and position of the placenta, or the amount of amniotic fluid present. When done to look for structural abnormalities in the fetus, it is best done from 16 to 20 weeks. Maternal serum screening is done around 15 to 20 weeks gestation to measure certain markers in mother's blood that may indicate the presence of certain birth defects or chromosome abnormalities.

In any of these forms of prenatal testing, pre- and post-test genetic counseling is important so that the limitations and risks of the procedures and tests are known. If testing reveals that the fetus is affected with a disorder, the couple can plan for the birth of an affected child or opt for elective abortion.

Newborn screening is one of the largest public health activities in the United States. It is aimed at the early identification of infants who are affected by certain genetic, metabolic, or infectious conditions for which early diagnosis and treatment are available. Approximately 4 million newborns are screened each year and 3,000 babies each year in the United States are found to have severe disorders detected. States test blood spots collected from newborns for 4 to over 30 metabolic and genetic diseases, such as phenylketonuria, hypothyroidism, galactosemia, sickle cell disease, and medium chain acyl CoA dehyrogenase deficiency. The goal of this screening is to identify affected newborns quickly in order to provide treatment that can prevent mental retardation, severe illness, or death.

Ethical Issues  (back to top)

Reproductive genetic testing raises a number of ethical issues. Of particular significance is the fact that the results of genetic testing on embryos or fetuses are used to make decisions about whether to continue or terminate a pregnancy, or to decide which embryos will be used to initiate a pregnancy. American’s have deeply held and widely varied views about the moral status of the embryo and the fetus. One’s view of prenatal diagnosis and other reproductive genetic testing is likely to be influenced by those beliefs, and the public discussion of these new technologies reflects the controversy inherent in the topic.

Although prenatal diagnosis has been routinely offered to women at risk of having a baby affected with a condition that can be prenatally detected for nearly 20 years, some are concerned that the ability to eliminate potential offspring with genetic conditions contributes to making society overall less tolerant of disability. Others have argued that prenatal diagnosis is sometimes driven by economic concerns because as a society we have chosen not to provide affordable and accessible health care to everyone. Thus, prenatal diagnosis can save money by preventing the birth of disabled and costly children. For reproductive genetic procedures that may involve greater risk to the fetus, concerns remain about whether the diseases being averted warrant the risks involved in the procedures themselves.

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Modified from text provided by the National Human Genome Research Institute.

Posted November 2002