What is genetic testing?
Genetic tests reveal whether a person carries gene mutations associated with certain inherited diseases. The tests involve analyzing blood, tissue, or amniotic fluid for evidence of genetic abnormalities. Tests available to couples who are planning a family include:
Carrier screening is a blood test performed on healthy adults to determine whether they carry a recessive gene mutation that could cause disease in their children.
What does it mean to be a carrier?
A carrier is a person who has only one copy of a gene mutation for a recessive disease. A carrier generally has no disease symptoms but risks having an affected child if her partner is also a carrier.
When is carrier screening performed?
Couples planning a family should ideally be screened before they conceive, or as soon as possible after conception, to allow all available reproductive options to be considered.
Because advances in genetic medicine occur quickly, couples should inquire about screening options before every pregnancy.
What does carrier screening involve?
A blood sample is taken from one or both partners and sent to a lab where the blood serum or DNA is examined for evidence of genetic mutations.
How long do results take?
Results of carrier screening are usually available in 2-[x] weeks.
[question: does the time it takes to get results vary widely depending on the disorder?]
What do the results mean?
If one partner is a carrier of an autosomal recessive genetic disorder but her partner is not, their children will not be affected by the disease. However, there is a 25% chance with each pregnancy that the child will inherit the defective gene and be a carrier.
[autosomal recessive inheritance chart: one parent a carrier]
If both partners are are found to be carriers of the same disorder, there is a 25% risk with each pregnancy that the fetus will be affected by the disease. The chances are 50% that the child will be a carrier and 25% that the child will inherit no mutation.
[autosomal recessive inheritance chart: both parents carriers]
If a gene mutation is detected on one of the woman's X chromosomes, there is a 50% chance that her male children will inherit the disorder. A male can't be a carrier of a recessive X-linked disorder because he has only one X chromosome, inherited from his mother. Males who inherit an X-linked condition such as Fragile X Syndrome are generally affected and females are unaffected carriers (though sometimes female carriers may be mildly affected, depending on the specific disease and mutation).
[X-linked inheritance chart: mother carrier]
If both partners are identified as carriers of the same disease, or if the mother carries an X-linked condition, a genetic counselor can provide more information and advice about available options.
For couples who are already pregnant, prenatal diagnostic tests are available to discover whether the fetus is affected.
Amniocentesis and chorionic villus sampling (CVS) are tests used to diagnose the fetus's state by analyzing fluid or tissue from the mother's uterus. Both are outpatient procedures that can determine with a high degree of accuracy whether the child of carrier parents is healthy.
Prenatal testing is also used to diagnose chromosomal disorders such as Down's Syndrome.
Amniocentesis has been widely used to detect genetic and chromosomal abnormalities since the early 1970s. Amniocentesis is performed in the second trimester, typically between the 15th and 18th weeks of pregnancy.
Using ultrasound as a guide, the physician inserts a thin needle into the uterus through the abdomen and draws out a small amount of amniotic fluid, which is then analyzed for evidence of genetic or chromosomal disorders. The procedure takes about 15 minutes.
Amniocentesis results are usually available in 1 to 2 weeks.
Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS) is a somewhat newer procedure with accuracy rates comparable to amniocentesis. Unlike amniocentesis, CVS can be performed in the first trimester, between the 10th and 12th weeks of pregnancy, allowing parents to get an earlier diagnosis.
CVS involves analysis of the chorionic villi, which are tiny branchlike projections of the placenta. It can detect the same recessive conditions as amniocentesis.
Using ultrasound as a guide, the physician inserts a catheter or thin needle into the uterus through either the abdomen or the cervix, depending on the position of the placenta. A small sample of the placenta is removed and analyzed for evidence of genetic or chromosomal abnormalities. Obtaining the sample takes only a few minutes, and the entire procedure takes about half an hour.
Results for CVS take about a week, though sometimes preliminary results are available earlier.
Are CVS and amniocentesis painful?
Both tests are performed under local anasthesia. Many women report no discomfort, while others experience mild cramping during or after the test.
Is prenatal testing safe?
Complications from CVS and amniocentesis are rare. Both procedures carry a very small risk of miscarriage or maternal infection. Studies show the risk is lowest when a highly experienced practitioner performs the tests. A genetic counselor can give you a referral to a physician or medical center with extensive experience in prenatal diagnosis.
Understanding the results of prenatal diagnostic testing
Disease inheritance is complex. A single gene can mutate in hundreds of different ways. Some mutations may cause mild symptoms, others severe. Your genetic counselor will analyze your fetal test results and help you understand what they mean.
Predictive testing, also known as presymptomatic testing, looks for genetic alterations associated with multifactorial diseases such as cancer or diabetes. Many people want to learn about their own genetic predispositions regardless of whether they are planning a family.
Predictive screening is not diagnostic; it can't tell you whether or not you will develop a disease. It only tells you whether you carry a gene mutation that puts you at higher risk for that disease than the general population.
People with a family history of illness often find it helpful to know their inherited risk so that they can make better decisions about their lifestyle and healthcare. A negative result can provide peace of mind, while a positive result can help you and your healthcare provider take steps to prevent the disease. Predictive screening also lets you know what risk factors you might pass on to your children.
Rapid advances in genetic medicine are constantly improving doctors' ability to detect mutations and diagnose disorders. However, genetic testing does have limitations:
- Genetic tests offer only a diagnosis, not a cure.
- Genetic testing cannot always predict the severity of a disease.
- Lab errors are rare but can occur. It is important to choose a reputable testing facility.
- There are still many genetic disorders for which tests have not yet been developed.
- Genetic test results are not straightforward, and interpreting them can be challenging even for a trained medical professional.
- Predictive testing for mutations associated with multifactorial diseases such as cancer won't tell you whether or not you will get the disease. It will only tell you about your genetic risk factors.
- No test is 100% definitive. Accuracy is generally high but varies depending on the disorder. To find out the detection rate for a specific condition, see our dictionary of genetic diseases.