Planning to have children?
Inherited genetic diseases cannot be cured, but they can be prevented. Carrier screening is a simple blood or saliva test that will reveal whether you or your partner carry gene abnormalities that could cause disease in your offspring.
Most genetic disorders affecting children are recessive, meaning they develop only when two copies of a defective gene are inherited. Parents who have a single copy of a recessive mutation are carriers and are unaffected by the disease. However, parents who are both carriers for the same condition can pass that condition on to their child if the child inherits an abnormal gene from each of them.
Genetic diseases are often fatal or severely debilitating. Because carriers have no symptoms, screening is the only way to know your risk of having an affected child.
When is carrier screening done?
Ideally you should be screened prior to starting a family, but the tests can also be done after conception, preferably as soon as you learn of your pregnancy.
Thanks to advances in genetic research, an increasing number of recessive genetic disorders can be detected through carrier screening. Many of these disorders are rare in the general population but more common in certain ethnic groups.
Cystic Fibrosis, Spinal Muscular Atrophy and Fragile X syndrome are three genetic disorders that occur more frequently in the general population. It is recommended that carrier screening for these disorders be offered to all prospective parents regardless of ancestry.
Whether you should consider additional tests depends on your ethnic background and family medical history.
- Ashkenazi Jews have a higher carrier frequency for Tay-Sachs and 16 other genetic diseases. The Ashkenazi panel was developed to screen for all these disorders with one blood test. (90% of American Jews are Ashkenazi.)
- People of Irish, British, French Canadian, or Cajun background have an increased risk of Tay-Sachs.
- Certain genetic disorders affecting the red blood cells are more common among people of Asian, African, or Mediterranean descent.
- A family history of miscarriages, children with developmental delays, or children who died young could indicate a genetic disorder running in your family.
If any of these risk factors applies to you or your partner, you should ask your physician about carrier screening, preferably before you become pregnant. Genetic diseases are rare in the general population, and the subject may not come up in conversations with your physician unless you mention it. Make sure you know your family medical history and tell your physician about your risk factors.
Generally both parents are tested. Depending on your particular sitation, you and your partner may be screened simultaneously or one at a time. Your healthcare provider takes a sample of your blood and sends it to a lab where it is analyzed for evidence of genetic abnormalities. Results take approximately 2 weeks.
If one parent tests positive for a an autosomal recessive disorder but her partner tests negative, the couple's child will not be born with the disease, though the child still has a 50% of being a carrier.
If the woman tests positive for an X-linked disorder such as Fabry Disease, her child has a 50% chance of inheriting the gene mutation for that disorder, since all children inherit one X chromosome from their mothers. A male child of a carrier mother will typically be affected by the disorder and a female child may or may not show symptoms (depending on the disease and/or mutation). Prenatal diagnostic testing is therefore recommended for pregnant women who are carriers of X-linked disorders.
What if we are both carriers?
If you and your partner are both carriers of the same recessive disease, there is a 25% risk that your child will be born with that disease. There is a 50% chance that your child will be a carrier with no symptoms of the disorder, and a 25% chance that your child will not inherit the gene mutation from either parent.
A genetic counselor can answer questions about your specific condition and advise you on the various family planning options available to you. Reproductive technologies such as in vitro fertilization (IVF) with preimplantation genetic diagnosis (PGD) have enabled many carrier couples to have healthy children. Other couples choose to adopt, or to become pregnant and have prenatal diagnostic testing.
Your family planning decisions are a personal matter. A genetic counselor will provide support and information to help you make informed choices, but only you and your partner can decide what is best for your family's health.
What is prenatal diagnostic testing?
If you are already pregnant when you learn that you and your partner are both carriers of the same genetic disorder, remember that the odds are on your side. There is a 75% chance that your child will be born without the disease.
The next step is prenatal testing to determine whether the fetus is affected. The fetus's status can be diagnosed with a high degree of accuracy through either chorionic villus sampling (CVS) at 10-12 weeks of pregnancy or amniocentesis after the 15th week.
Results take approximately 1 to 3 weeks. Your physician or a genetic counselor can help you determine which test is appropriate for your particular situation.
What is prenatal diagnostic testing?
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 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.
Amniocentesis results are usually available in 1 to 3 weeks.
Chorionic villus sampling (CVS)
Chorionic villus sampling (CVS) is a prenatal testing method 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 genetic diseases 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.
Results for CVS take approximately 1 to 3 weeks.
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. Some gene 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 for multifactorial diseases
Predictive 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.
Limitations of genetic testing
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.