Prenatal screening includes ultrasounds and tests on the pregnant woman’s blood. These tests are usually done in the first and second trimester. Screening tests can determine if there is a higher or lower chance for some genetic conditions, such as Down syndrome, trisomy 18, and spina bifida. For example, a pregnant woman might be told that her odds for having a baby with Down syndrome are 1 in 500. This number depends on her individual test results, and a medical professional can help interpret what the number means. However, screening tests do not give a “positive” or “negative” result. In addition, these screens can miss some pregnancies where a condition is actually present (“false negatives”). They can also incorrectly show high chances for a condition when none exists (“false positives”). (1) These screens can accurately suggest a higher chance of Down syndrome about 50-95% of the time, depending on the methods used and the condition, and they show “false positives” in about 5% of cases. (2) No blood test currently offered can give a definite “yes” or “no” answer even though they can often identify those with increased chances.
Ultrasound screens for some genetic conditions as well. For instance, a measurement at the back of the fetal neck is known as the nuchal translucency (NT) and if increased in thickness in the first trimester, there is a higher chance of a chromosome condition such as Down syndrome. In the second trimester, there are measurements and findings on the ultrasound that can show an increased or decreased chance of a chromosome condition, but there is no way to find a prenatal chromosome condition for sure based on ultrasound findings alone. For instance, a fetus with shorter than average femur (thigh bones) on ultrasound may have an increased chance of Down syndrome. However, most babies with shorter than average thigh bones do not have Down syndrome, and some babies with Down syndrome have average length thigh bones. Ultrasound is a very good tool for detecting most neural tube defects such as spina bifida. In fact, greater than 98% of neural tube defects can be seen on ultrasound in the second trimester.
There are also newer non-invasive prenatal screening tests that can be done after 9 or 10 weeks. These tests have the highest detection rates among all screening tests, specifically for women who have already learned their pregnancy has greater chances for a genetic condition. These tests look at pieces of genetic material in the woman’s blood to find chromosome conditions, such as Down syndrome, trisomy 13 and 18, and, sometimes, sex chromosome and other genetic conditions. These tests are still considered screening tests because false negatives and false positives do occur. The accuracy is improved but varies depending on the condition. So, diagnostic testing is recommended for those who want to know for sure. In addition, these tests do not detect many other genetic conditions or factors that might be present in a pregnancy. These tests also might not be as reliable in some situations, such as with twins or multiples. The results given by the testing companies can be very different from each other and should be reviewed with the patient by a medical professional.(2)
Dr. Mary Norton recommends that the non-invasive screening tests are most helpful as a secondary screen.
More information is also available in a fact sheet about the use of non-invasive prenatal testing (NIPT) developed by he National Coalition for Health Professional Education in Genetics (NCHPEG) and National Society of Genetic Counselors (NSGC).
- ACOG Practice Bulletin No. 77, January 2007. Screening for Fetal Chromosomal Abnormalities. Obstet Gynecol 2007;109:217–27.
- Anthony R. Gregg, S.J. Gross, R.G. Best, K.G. Monaghan, K. Bajaj, B.G. Skotko, B.H. Thompson, M.S. Watson. ACMG statement on noninvasive prenatal screening for fetal aneuploidy. Genet Med 15: 395-398; April 4, 2013; doi:10.1038/gim.2013.29
- Morrow R.J., McNay M.B., Whittle M.J. Ultrasound detection of neural tube defects in patients with elevated maternal serum alpha-fetoprotein. Obstet Gynecol 1991 Dec;78(6):1055-7.
For more information about prenatal testing and prenatally diagnosed conditions, click below:
Understanding Chromosome Conditions
Trisomy Conditions (Down syndrome, Trisomy 18, and Trisomy 13)
Sex Chromosome Conditions (Turner syndrome, Klinefelter syndrome)
Neural Tube Defects (Spina Bifida)
Microdeletions (Angelman/Prader-Willi, 22q, Wolf-Hirschhorn, Cri-du-Chat, Jacobsen, and Williams syndrome)
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