Genetic testing during pregnancy, or prenatal genetic testing, looks for possible genetic disorders in a fetus. Genetic disorders are due to missing or extra chromosomes or inherited changes to the DNA.
For example, an extra copy of chromosome 21 causes Down syndrome. This extra chromosome happens randomly and is typically not inherited from a parent. By contrast, cystic fibrosis is an inherited disorder that occurs because a parent passes on a genetic mutation.
There are two types of genetic testing offered during pregnancy: prenatal screening tests and prenatal diagnostic tests. Parents-to-be get these tests at different times and for different reasons.
Remember that any pregnant person can get genetic testing, but it is always optional. Some parents want this information, and others don’t. It can cause both anxiety and peace of mind.
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Types of Genetic Screening Tests During Pregnancy
A prenatal screening test tells you the chances that your baby has a chromosomal or other genetic disorder. Some tests also look for the likelihood of other conditions, including neural tube defects (problems with the brain and spine). These tests are non-invasive, meaning they do not pose any potential risk to the developing fetus.
In the first trimester, usually between 10 and 13 weeks, a pregnant person can opt for a blood test and an ultrasound. This test is sometimes called the “first trimester screen.” Together, these tests look for the likelihood that a baby may have Down syndrome or other types of chromosomal disorders. Doctors can also assess the baby’s risk of having certain heart, spine, or abdominal wall problems during the ultrasound.
In the second trimester, the “quad screen” is a blood test that measures four substances in the pregnant person’s blood. This test looks for certain chromosomal disorders and neural tube defects. For parents who want this testing, doctors recommend doing it between 18 and 22 weeks of pregnancy. This testing is a good option for parents who did not get the chance to do the first trimester screen.
A positive blood screen test means that your baby has a higher risk of having the disorder (compared to the general population). It does not mean they definitely have it. Likewise, a negative result means the risk is lower. For more definitive answers, you will need a diagnostic test.
If you did the first trimester screen, you can opt for an additional blood test between 15 and 22 weeks of pregnancy to determine the baby’s risk for having neural tube (brain and spinal cord) abnormalities, such as spina bifida.
For pregnant patients over the age of 35, the risk of Down syndrome increases, and a test called non-invasive prenatal testing (NIPT), or cell-free fetal DNA is preferred over the first trimester screen. This test can be done as early as 10 weeks of pregnancy up to the end of pregnancy. This test looks for the fetal DNA that circulates in the pregnant patient’s blood and is extremely accurate. Because it looks at your baby’s DNA, it can tell you the fetal sex if you want to know.
The second-trimester ultrasound also checks for brain, spine, heart, limbs, stomach, and face problems. If your doctor suspects a condition because of an ultrasound finding, they’ll send you for further testing.
Types of Prenatal Genetic Diagnostic Tests
A diagnostic test can tell you, with near certainty, if your baby has a chromosomal or genetic disorder.
If desired, the doctor can request testing for inherited genetic disorders. They need to specify what gene to test. Parents usually do this testing because they or their partner carry a certain gene, like the cystic fibrosis gene.
The two main types of genetic diagnostic tests doctors do during pregnancy are:
- Amniocentesis: The doctor uses a thin needle to get a small amount of amniotic fluid, which they can test for fetal genes. The typical time frame is between 15 and 20 weeks, but you can have an amniocentesis up until delivery.
- Chorionic villus sampling (CVS): This test analyzes a tiny sample of placental tissue. You can do it earlier than amniocentesis. Doctors usually perform between 10 and 13 weeks.
Both amniocentesis and chorionic villus sampling (CVS) come with the low risk of miscarriage. Your doctor will review the risks of each of these tests with you before you decide to undergo diagnostic testing.
Is Prenatal Genetic Testing Right For Me?
Doctors offer all pregnant people genetic testing. Some people say yes; others say no. There is no right or wrong answer.
A genetic counselor can help you work through the pros and cons. Thinking ahead to what you will do with the information can be helpful. Knowing ahead of time about a disorder can help you prepare for medical care your child may need shortly after birth.
While some screenings are standard, like ultrasound in the second trimester, genetic testing during pregnancy is a choice. If you’re uncertain, a discussion with your doctor is the best place to start. They can give you a fuller picture of the risks and benefits of genetic testing.
Prenatal Genetic Diagnostic Tests. American College of Obstetricians and Gynecologists. Link.
About UPMC Magee-Womens
Built upon our flagship, UPMC Magee-Womens Hospital in Pittsburgh, and its century-plus history of providing high-quality medical care for people at all stages of life, UPMC Magee-Womens is nationally renowned for its outstanding care for women and their families.
Our Magee-Womens network – from women’s imaging centers and specialty care to outpatient and hospital-based services – provides care throughout Pennsylvania, so the help you need is always close to home. More than 25,000 babies are born at our network hospitals each year, with 10,000 of those babies born at UPMC Magee in Pittsburgh, home to one of the largest NICUs in the country. The Department of Health and Human Services recognizes Magee in Pittsburgh as a National Center of Excellence in Women’s Health; U.S. News & World Report ranks Magee nationally in gynecology. The Magee-Womens Research Institute was the first and is the largest research institute in the U.S. devoted exclusively to women’s health and reproductive biology, with locations in Pittsburgh and Erie.