If you’re trying to get pregnant, you may have already gathered important personal health information like your family’s medical history. Your doctor will likely also offer certain tests and screenings, including what’s known as a carrier screening. This test, which can be done before you conceive or during pregnancy, can tell you if you and your partner are at risk of passing along certain genetic diseases to your children that you may not have yourself, like Tay-Sachs or cystic fibrosis.
What is a genetic disease?
Everyone has two sets of DNA: one from Mom and one from Dad. When a sperm (carrying Dad’s DNA) fertilizes an egg (containing Mom’s DNA), those two sets of DNA combine to make a new mixture — the unique genetics of your baby. Your baby’s genetics will determine your child’s hair and eye color, height and all sorts of other traits.
A genetic disease is when one or both parents' sets of DNA have a mutation (a change from the usual), which could make a baby more prone to developing health conditions. Some mutations are harmless, some only slightly boost the risk of a condition, and others can cause more serious diseases or developmental problems starting at birth.
What does it mean to be a genetic “carrier”?
Just about everyone carries a gene for at least one genetic disorder — even if it’s never shown up in a family history. If you have a mutation in just one set of genes, you’re what’s known as a "carrier": You’re carrying the genes for a genetic disorder but have no signs of the disease.
Rarely, a condition (like Huntington’s disease) can be caused by a mutation in just one set of DNA. But in most cases — including cystic fibrosis and sickle cell disease — it takes a mutation in both sets of DNA, one from Mom and one from Dad, for a child to actually be affected by a genetic disease.
So what’s a carrier screening?
A carrier screening tests whether you or your partner is a carrier for certain genetic diseases. It’s a blood test, which requires a quick prick to draw some blood from your arm. If both you and your partner are carriers, and you both pass the mutation-carrying DNA along to a baby, the baby could end up with a full-fledged version of the disease — so this test looks at the odds of that occurring.
Who should get a carrier screening?
Parents can be tested for some of genetic disorders before (which is preferable) or during pregnancy, thanks to genetic testing. In almost all cases, testing is recommended for one parent. Testing the second parent only becomes necessary if the first tests positive.
Carrier screenings recommended for all parents
In the past, the American Congress of Obstetricians and Gynecologists (ACOG) only recommended carrier screenings to parents of certain ethnic or geographic backgrounds considered more at risk of specific disorders. However some conditions are not limited to one ethnicity. What’s more, many people are of mixed backgrounds, which means it can be hard to make recommendations based on an ethnic or geographic basis. And, although they’re still very rare, some genetic conditions are common enough that practitioners should offer to screen for them in every patient. That’s why ACOG recently released new guidelines for women who are planning to have a child or are already pregnant. The group now recommends that all women should get screened for cystic fibrosis and spinal muscular atrophy (SMA) as well as a receive a complete blood count to screen for risk of hemoglobinopathies (which includes sickle cell disease and thalassemias).
Carrier screenings for at-risk parents
Because some people are more likely to carry specific genetic mutations, your doctor may recommend a number of other screenings based on your family heritage and/or medical history:
- Eastern and Central Eastern (Ashkenazi) Jewish descent: Tay-Sachs disease, Canavan disease and familial dysautonomia; there are a number of other disorders some practitioners screen for including Bloom syndrome, Maple syrup urine disease and Niemann-Pick disease, among others
- French-Canadian or Cajun: Tay-Sachs disease
- Ovarian insufficiency syndrome: Fragile X syndrome; the screening should also be offered to women with a family history of fragile X-related disorders
- Other family history of disease: Anyone who has a family history of genetic diseases — a cousin who had Tay-Sachs disease, for instance — should be screened for those diseases as well, as they’re more likely to be a carrier
Your health care provider should have a standard approach to screening for additional disorders that he or she will discuss with you in advance.
Expanded carrier screening
Expanded carrier screening now enables all couples, regardless of their ethnic or geographical profile, to test for a broad array of genetic conditions before conceiving. It can screen for the carrier gene of hundreds of diseases, giving you the power of knowing whether you and your partner are at risk of passing along any of these genetic conditions to a baby you conceive together.
That said, just because the test is available does not mean couples should be screened for all the diseases out there. ACOG has specific recommendations for which disorders practitioners should include in an expanded carrier panel. Conditions should occur in at least 1 in 100 people, reduce the quality of life, impair cognitive or physical abilities, require surgical or medical intervention and have an onset in childhood. Here are a few of the most common diseases expanded carrier screenings test for:
- Cystic Fibrosis
- Familial Hyperinsulinism
- Fanconi Anemia Group C
- Fragile X Syndrome
- Gaucher Disease
- Glycogen Storage Disease Type 1a
- Maple Syrup Urine Disease Type 1A/1B
- Neimann-Pick Disease Type A/B
- Sickle Cell Disease
- Spinal Muscular Atrophy (SMA)
- Tay-Sachs Disease
Again, your practitioner likely has a standard expanded carrier screening panel that’s offered to all patients, which he or she should adjust for any risks specific to you and your partner. If you’re interested in testing for other specific diseases, talk to your practitioner about your and your partner’s risk factors, as well as how you’ll use the information about each disease. If you have further questions, a conversation with a genetic counselor can be helpful.
Before you get carrier screening
Both ACOG and the American College of Medical Genetics and Genomics (ACMG) agree that all couples should be offered the option of carrier screening, should they choose. To reduce the potential emotional downside of screening, experts recommend that couples have a discussion with an OB/GYN and/or genetic counselor before getting the testing to make sure they understand what’s being tested for. Any discussion about carrier screening should also include a conversation about what you would do if these results come back positive. You can choose to opt out of any or all carrier screenings or request not to receive certain results. Ultimately, it’s up to you and your partner to make an informed decision about what’s best for you and your soon-to-be growing family.
When is carrier screening done?
Genetic carrier screening can be done when you’re just in the planning stages of starting a family, while you’re actively trying to conceive or once you’ve gotten a positive pregnancy test. That said, if you're interested in the test, the earlier you get it done, the more likely doctors can do something if they do detect that you’re carrying a mutation.
How is it done?
Once a sample of your blood is taken, DNA will be isolated in a lab from your blood cells, so that any mutations that are being tested for can be detected. Because most hospitals have to send your blood to an off-site laboratory to do carrier testing, it will likely take between one to two weeks to get your results.
What are the odds of our baby having a disease if we test positive?
Even if both you and your partner test positive as carriers of the same genetic mutation, there’s still only a 1 in 4 (or 25 percent) chance that your baby will have the disease. That’s because each of you has two sets of DNA. Since you are carriers and don’t actually have the disease, that means you each have a second, healthy copy. If the baby inherits the healthy copy from one or both of you, he won’t have the disease (although your child may be a carrier).
Also know that even if you discover your baby is likely not at risk for the diseases your doctor screens for, carrier screening still can’t identify every at-risk person. In addition, current tests don’t cover all genetic mutations. There are many hundreds of diseases that doctors can’t yet test for, though the science is improving all the time.
Is there anything we can do if we test positive?
The most powerful time to get information from carrier screening is before you’re pregnant. If you and your partner do test positive as carriers of a genetic disease, you may choose to work with fertility doctors to discuss your options. For example, technology now exists that can test the DNA of embryos created via IVF to see whether they have a particular mutation (this is called preimplantation genetic diagnosis). A positive result also enables you and your partner to consider sperm donation and other nontraditional routes to starting a family.
If you get a carrier screening done after you’re already pregnant, a positive result can mean more tests to see whether your baby is affected. A positive result on one of these tests can be valuable to help you think ahead and plan financially. It could also lead to early treatments for the disease before the baby is born as well as arrange for special care during and after birth.
And since genetic disorders do tend to run in families, ACOG suggests that you should inform relatives of positive results so they can decide whether or not they would like to be screened as well.
Even if you don’t talk to a genetic counselor before proceeding with carrier screening, many health care practitioners will refer you to one to help you interpret your results and decide what to do with any information you obtain. As more and more tests become available, understanding what they mean can be confusing — genetic counselors are trained in helping you sort through the available information to make sense of it.
If your results are fairly straightforward, and your obstetrician is comfortable with genetic screening, she may perform this counseling on her own. But if you’re confused about anything or want to talk through your options with someone else, it’s always your right to ask to see a genetic counselor — most hospitals have one on staff and your insurance should cover your visit.
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