The different types of prenatal genetic testing
These tests check for genetic problems before a baby is born. They might sound scary but provide valuable information.
The thought of prenatal genetic testing can give pregnant women pause, just as they are excited about having a baby and imagining their little one’s future. When the doctor mentions an alphabet soup of scary-sounding tests for genetic problems, it’s easy to get nervous. Knowing what to expect — about these tests and your baby’s health — can reduce your fear.
Here’s a breakdown of different types of prenatal genetic testing a woman should be offered during pregnancy:
Quad or triple screen
This is an older test typically done between 15 and 20 weeks of pregnancy. During this test, a mother’s blood is analyzed for three or four different hormone levels; hence, the quad or triple part of the name. The results determine if a pregnancy is at an increased risk for Down syndrome (trisomy 21), trisomy 18 or spina bifida. This is not a diagnostic test. If the results are abnormal, it does not mean your baby has a problem. The next step is a referral for additional testing. The rate of false positive (the test is abnormal but everything is normal) is about 5 percent. About 80 percent of babies with Down syndrome will be detected by this test.
First trimester screening/ultrascreen
First-trimester screening is a newer test shown to be more accurate than the quad/triple screen. This test can also occur sooner in pregnancy. This test combines an ultrasound measurement of the nuchal translucency (fluid in a fold of skin behind the baby’s neck) with a maternal blood sample. It analyzes two hormone levels in the mother’s blood and calculates risk of Down syndrome, trisomy 18 and trisomy 13. It is typically done between 11 and 13 weeks of pregnancy. This is also a screening test, so it cannot determine if a baby has one of these conditions. The detection rate is about 85 percent and the false positive rate is about 5 percent. Since this test does not include screening for spina bifida, a second maternal blood test is recommended at 15 to 20 weeks.
Non-invasive prenatal screening/cell free DNA
This is the newest addition to genetic testing options during pregnancy. The American College of Obstetricians and Gynecologists recommends this test for all women with an increased risk of a pregnancy affected with a chromosomal abnormality. If the first trimester screening or quad/triple screen is abnormal, this is often the next step. Some women will be offered this test first, based on risk factors such as age, health history or pregnancy history.
Multiple companies perform this test, and you may hear women refer to it by brand names such as MaterniT21, Panorama or Harmony. Each company offers slightly different screening in their product, but the method for testing is the same. During pregnancy, a small amount of genetic information (DNA) from the baby is found in the mother’s blood. This test analyzes these DNA fragments and looks for patterns of more or less genetic information than expected. This screens for conditions such as Down syndrome, trisomy 18, trisomy 13 as well as a number of different chromosomal abnormalities. The gender of the baby can also be determined — however this test should NOT be used only for that purpose. The rate of false positives is 0.1 to 0.2 percent and the detection rate is 99 percent. It is important to know that this is also a screening test and is not a diagnosis for these conditions.
There are also options for diagnostic tests, such as chorionic villus sampling and amniocentesis. These are invasive tests that come with risks.
Genetic counselors are available at Vanderbilt and most doctor’s offices to help you wade through the sea of options and point you to the right tests. Understanding your prenatal genetic testing options during pregnancy will help you ask questions and determine the right tests for you and your family.
This post was written by Bethany Sanders, a certified nurse midwife who cares for women at the West End Women’s Health Center. When not attending births or measuring pregnant bellies, she talks about cloth diapers with anyone who will listen.