What is genetic testing (carrier screening)?
Genetic testing is when a blood test is given to prospective or expecting parents to look for abnormal genes that can lead to certain diseases in their baby. Most genetic diseases are known as "recessive disorders," which means that each parent needs to pass along an affected gene to the baby in order for the child to be affected. In other words, if you screen positive for a genetic abnormality but your partner does not, your child will not inherit the condition. And even if you both screen positive, there's only a 25 percent chance your baby will have the disease.
You can get a carrier screening at your ob-gyn's office with a single blood test; you typically receive the results in two weeks. Most often, a woman is tested first (only because you're more likely to see your doctor regularly if you're in trying-to-conceive mode) and if those results don't show a problem gene, her partner usually doesn't need to be tested.
When should you get genetic testing?
Genetic testing is ideally done before you start trying to get pregnant, says Angela Trepanier, MS, CGC, co-director of the genetic counseling program at Wayne State University and president of the National Society of Genetic Counselors. "But because so many pregnancies are unplanned, many couples get screened early in pregnancy," she says.
Getting screened before you try to get pregnant can give you reassurance (if you or your partner is not a carrier, it's one less thing to worry about when do you get pregnant) or can help you make an informed game plan for pregnancy. If it turns out that you are both carriers, you can be prepared for and bone up on what it means to have a baby with the genetic condition, choose to learn about certain prenatal tests to check whether your baby's healthy, or you can consider other options like egg or sperm donation or adoption.
Getting tested once you become pregnant (if you hadn't done so ahead of time) can help you and your doctor decide the right prenatal tests for your baby, and what to look for if you choose to have them. If you know that your baby's at an increased risk for having cystic fibrosis or sickle cell disease, for instance, your doctor can look for those conditions specifically through either a CVS (chorionic villi sampling) or amniocentesis.
Who should get genetic testing?
Your ethnic background and family history are the main factors that determine whether or not genetic testing is right for you. "That's because the diseases for which genetic testing is currently offered tend to be more common among certain ethnic groups," says Trepanier. Here's an example of groups who are more likely to have certain genetic diseases.
- European Caucasians and Eastern European Jews face an increased risk of cystic fibrosis.
- Eastern European Jews also face an increased risk of conditions like Tay-Sachs, Canavan, and familial dysautonomia.
- African Americans and people of Mediterranean descent (Greeks, Turks, Italians) face an increased risk of conditions like sickle cell anemia and thalassemia. People of Southeast Asian descent also face an increased risk of thalassemia.
- French Canadians face an increased risk of Tay-Sachs and cystic fibrosis.
Though there are many other recessive genetic diseases, they are not routinely checked for because they're either very, very rare or because the right tests have not been developed yet, says Trepanier.
What are some of the most common genetic diseases?
- Cystic fibrosis is a life-threatening condition that causes lung damage and digestive problems. About 30,000 people in the U.S. have the disease, which mostly affects Caucasians of Northern European descent. Organizations like the National Institutes of Health, the American College of Obstetricians and Gynecologists, and the American College of Medical Genetics have recommended nationwide genetic testing for CF for all adults since 2002. According to a recent study in the New England Journal of Medicine, the number of babies born with cystic fibrosis in Massachusetts dropped by about 50 percent in the four years after this recommendation was made and testing came into wider use compared to the four years before.
- Sickle cell disease, most common in people of African and Mediterranean backgrounds, causes a blood disorder that leads to anemia, a weakened immune system, and other health complications.
- Thalassemia is another blood disorder common to people of African and Mediterranean descent. It typically causes anemia and bone growth and liver problems; in severe cases, some babies born with the condition may not survive.
- Tay-Sachs Disease, which mainly affects people of French Canadian and Eastern European Jewish descent, is a disorder of the central nervous system that's usually fatal in early childhood. Eastern European Jews also face an increased risk for another nervous system disorder called Canavan disease as well as a number of other conditions including familial dysautonomia, familial hyperinsulinism, and Gaucher disease. Your doctor can screen for all of these conditions at the same time.
- Fragile X Syndrome, a condition that can cause developmental problems, including learning disabilities and mental retardation, is not linked to a specific ethnic background. Reviewing your family's health history with a doctor or genetic counselor may help you decide whether you should be screened for Fragile X.
What does genetic testing tell you?
Genetic testing results give you an idea of your risk for passing along genes that can trigger certain diseases in your baby.
- If neither you nor your partner is a carrier, your baby will not inherit the condition.
- If you are a carrier, but your partner is not (or vice-versa), your baby will not inherit the condition.
- If you and your partner are both carriers, your child has a 25 percent chance of inheriting the condition.
While genetic testing can give you and your partner visibility into your health history and your chances for a healthy pregnancy and baby, it's not totally foolproof. "Some genetic tests, like cystic fibrosis, can only identify some of the mutations in the genes that cause the condition," says Trepanier. So even if you get negative results, there's still a slim chance you could be carriers and pass on the condition to your baby.
Your doctor may be able to recommend a genetic counselor, or you can find one through the National Society of Genetic Counselors (nsgc.org) or the American Board of Genetic Counseling (abgc.net).
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