Carrier Screening & Prenatal Genetic Testing for Birth Disorders
Pregnancy inherently carries many unknowns. (So does parenting, really!) What will your child look like? What will her personality be? Will he be born on his due date or will he wait to arrive a week later?
While many questions about your child’s future can’t be answered, some questions can. Questions like, “What is the sex of my baby?” “Is my baby developing as expected?” “Could I pass a genetic disorder on to my children?” “What are the odds that my baby will be born with any birth defects?”
Ultrasounds are one way that we can help you answer some of those questions, such as the sex of your baby or his/her growth and development. Read more about what is learned in ultrasounds during pregnancy.
Other questions about genetic disorders and risks for birth defects can be addressed through carrier screening and prenatal genetic testing for birth disorders.
At Madison Women’s Health, we offer carrier screening and genetic testing for birth defects or birth disorders at preconception appointments, during a woman’s first trimester and at her 20+ week scans. We do most of these important tests in-house so our patients can speak with a genetic counselor in a more familiar environment. It’s just another way we reduce stress and help our patients feel more comfortable and cared for at every stage.
What to expect when you meet with a genetic counselor
Whether you’re meeting with a genetic counselor for a preconception appointment or after you’re already pregnant, we will discuss your goals for the visit. We’ll go over your health history and concerns that you may have. Your genetic counselor will explain (in more detail than this article!) more about how genetics work. She’ll go over what can be tested at the first trimester and what is tested at later visits. If you’ve already received your test results, she will go through the results report with you to make sure you understand what they mean. You can ask questions at any time. Our goal is for you to feel comfortable with the information and resources you have available to you.
What is carrier screening
It’s important to understand what carrier screening is, why and when it’s done, and how it is different from testing for birth defects or birth disorders.
Carrier screening, also called pre-conception or prenatal genetic testing, tests your DNA. Because it tests your DNA (not your baby’s), it can be done at any time, whether you are pregnant or not. It can determine whether you carry a gene mutation that would put you at higher risk of having a child with a genetic condition such as cystic fibrosis. (You may be healthy, but if you and your partner both have the mutation for a genetic disorder, there is a greater likelihood it could be passed on to your child.) Carrier screening involves testing a sample of your blood, saliva, or the inside of your cheek.
Why get genetic testing
People choose to have carrier screening done for different reasons:
- If they have a family history of a particular genetic disorder;
- If they are of a specific ethnic background and want to rule out genetic disorders that are more common for that ancestry. (For example, people of Jewish descent are more likely to carry the gene for Tay-Sachs disease, people of African descent are more likely to carry sickle cell anemia, and people of Asian descent are more likely to be genetic carriers of Alpha-Thalassemia).
- If they have had one or more children with a genetic disorder.
- If they have no increased risk but would find negative results reassuring.
What to do if you get a positive test result
Genetic disorders are very rare. In fact, only about 3-4% of people are carriers for the most common disorders of cystic fibrosis and spinal muscular atrophy. Even if your test comes back positive for one of these genetic mutations, your child still isn’t guaranteed to have the disorder. Both you and your partner would need to be carriers of the gene for it to have a significant chance of affecting your pregnancy.
If you do carry a genetic mutation, then the next step is for your partner to be tested for that disorder. If both you and your partner are carriers of the same genetic mutation, then there is still only a 25% chance of passing the disorder on to your child.
Screen for genetic disorders at your preconception visit
Screening for genetic disorders during your preconception visit can give you options regarding pregnancy planning including:
- Choose to pursue pregnancy the traditional way. During the first trimester, we will discuss options for prenatal testing, if desired, and pregnancy management.
- Choose to get pregnant with a donor egg or sperm.
- Choose to get pregnant through IVF (in vitro fertilization) using pre-implantation genetic testing, which can be done once the embryo develops. Only implant the embryo(s) that does not carry the genetic mutation.
- Choose adoption as a way to grow your family.
(Insurance coverage varies for genetic testing based on your health history and a number of other factors.)
Genetic Testing vs. Testing for Birth Disorders
The easiest way to understand the difference between genetic testing (carrier screening) and testing for birth defects or chromosomal disorders is to look at who is being tested. With genetic screening, your own DNA is being tested. When testing for birth disorders, your baby’s DNA or development is being tested.
Testing for birth defects or birth disorders can be done once you are pregnant because they are specific to the baby you are carrying at that time.
Most Common Birth Disorders to Screen For
The most common birth disorders that OBGYNs test for are chromosomal disorders:
- Down syndrome
- Trisomy 13
- And Trisomy 18.
These disorders occur randomly when an extra chromosome is formed during the fertilization of the egg. These are most often screened for at the first trimester exam, between weeks 12 and 13. (Sometimes, the tests can be done as early as 10 weeks gestation if you have certain risk factors, such as a previous pregnancy with down syndrome or advanced maternal age (aka “geriatric pregnancy.”)
Typical Schedule of Prenatal Genetic Testing
Week 10 or Week 12-13:
- Down Syndrome, Trisomy 13, Trisomy 18, predicted fetal sex — blood test
- Nuchal translucency — ultrasound
- Spinal bifida — blood test
- Anencephaly — blood test
- Growth — ultrasound
- Physical development — ultrasound
- Common birth defects such a heart defect or cleft lip — ultrasound
- Fetal sex–ultrasound
- Others as needed based on family history
Accuracy of Prenatal Birth Disorder Testing
Screening for birth disorders is not 100% accurate. The tests can produce false positives and false negatives. Some tests are more accurate than others.
On average, carrier screens have a detection rate greater than 90%. The lab that Madison Women’s Health uses has a very high detection rate for first trimester screening. The detection rate for Down syndrome is around 98% and the detection rate for trisomy 13 and trisomy 18 is 95%.
NIPT (Non-invasive prenatal testing) is able to detect Down syndrome 98-99% of the time, and can detect trisomy 13 and trisomy 18 90-98% of the time.
Read about at-home genetic testing kits and whether they really work.
What to Do if Your Test Results are Positive
Until recently, patients didn’t have easy access to their medical records and tests results. Now that health records are electronic and easily accessible using MyChart, lab results are released to patients automatically. That means that you could see your test results at the same time as your OBGYN or genetic counselor — even before we have had the chance to call you about them.
When we call you with any abnormal or positive test results, our goal is to help you understand what the results mean, what kinds of options you have, and how you can plan. We expect that you’ll have many questions for us, and we’re prepared to give you the answers and guidance you’re looking for.
It’s important to understand that for many of the screenings, “positive” test results simply mean “increased risk” of having a disorder. After receiving positive test results, the next step you’ll take is to have more testing to confirm a diagnosis. You’ll talk to our genetic counselor to decide if and how you want to do confirmatory testing or if you want to simply wait until your baby is born.
If a diagnosis is made, then our next goal is to to provide education and support regarding your pregnancy decisions. We will share helpful resources and will set up meetings with healthcare providers and specialists. In some cases, we can even put you in touch with parents who have been in a similar situation.
Pros and Cons of Prenatal Testing for Birth Disorders
While there are some birth defects that can be treated in utero (fetal surgery can treat spina bifida, for example), most of the time, the baby cannot be treated until he or she has been born.
So is it better to test early for birth defects or to wait until the baby is born? Most of our patients choose to test, but others choose to wait. Let’s take a look at the pros and cons of prenatal testing.
Learning that your baby is at risk of being born with a birth defect could be upsetting or stressful, especially if there isn’t anything you can do to fix it. If the test was a false positive, meaning follow-up testing was normal, then you worried unnecessarily. Some women choose not to have the tests done to avoid the additional stress or worry.
On the other hand, learning they are at low risk for a pregnancy with a birth defect is one reason is one reason many of our patients choose to have these optional tests done. It gives them peace of mind during the other unknowns of pregnancy. If their tests come back positive for increased risk of a genetic disorder or birth defect, many appreciate the extra time they have to make their plans. For example, they may need to schedule surgeries, find additional help, or make some other changes in their home.
In the end, the decision is up to you — do you want as much information ahead of time as possible to make plans? Or do you prefer to live more in the moment with an optimistic or pragmatic perspective about the future?
Whatever your choice, we are here to support you, to educate you, and to help you make the best plans for your family.
Jill Ciske is a certified genetic counselor who has been with Madison Women’s Health since 2011. Jill received her undergraduate degree in Genetics from University of Wisconsin-Madison, went on to receive her Masters of Science in Genetic Counseling from the University of Minnesota, and has been certified by the American Board of Genetic Counseling since 1996.