A non-invasive maternal blood test that can detect certain fetal chromosomal disorders, including Down syndrome, early in pregnancy is gaining attention as a potential new method of prenatal screening. Currently, several organizations support its use in pregnant women at high risk of having a child with a disorder caused by an abnormal number of chromosomes. However, invasive diagnostic tests, such as chorionic villus sampling (CVS) and amniocentesis, are still needed to confirm the results.
The test, which was first introduced in October 2011, is called cell-free fetal DNA (cffDNA) and requires only a blood draw from a pregnant woman's arm. The blood sample is tested for fetal cell-free genetic material that circulates in her blood. Currently, the test screens for three genetic disorders: trisomy 21 (Down syndrome), trisomy 18 (Edwards syndrome), and trisomy 13 (Patau syndrome). The cffDNA test can be done as early as the tenth week of pregnancy, with results available in about one week.
A method of screening for fetal chromosomal abnormalities currently being used measures the amount of certain markers in a maternal blood sample combined with results from an ultrasound and is usually performed between 10 and 11 weeks of pregnancy (see First Trimester Down Syndrome Screen). It can correctly identify approximately 85% of women carrying a fetus with Down syndrome with a false positive rate of 5%. By comparison, studies of cffDNA have shown an ability to recognize 98% of cases of Down syndrome, with a false positive rate of less than 0.5%, although these studies were among high risk women only. The first trimester screen only assesses risk of Down syndrome and Edwards syndrome, whereas cffDNA screens for three trisomy disorders with the potential for more.
The American College of Obstetricians and Gynecologists (ACOG) released a position statement in December 2012 that called the new test highly accurate for screening but not for diagnosis. For now, ACOG says cffDNA testing should be offered, after pre-test counseling, only to pregnant women at increased risk for trisomy 13, 18, or 21. They include: women age 35 and older, women with a history of a pregnancy with a child with a trisomy, and women carrying a fetus with ultrasound findings indicating increased risk.
ACOG says the test should not be offered to low-risk women or women carrying multiple fetuses because it has not been sufficiently tested in those groups. Additionally, ACOG recommends that women with positive cffDNA results be referred for genetic counseling and be offered CVS or amniocentesis to confirm the diagnosis. The International Society for Prenatal Diagnosis and the National Society of Genetics Counselors have released similar statements.
As interest in cffDNA continues to grow, it is expected that research will explore the use of cffDNA testing among low-risk women and will increase the number of disorders the testing can detect. It is possible that cffDNA could become a reliable, non-invasive method for prenatal diagnosis of, not just screening for, a variety of chromosomal abnormalities. For now, however, it offers the potential benefits of improved diagnostic accuracy for trisomy 21, 18, and 13 over existing screening tests among women at high risk of carrying a baby with one of these abnormalities.
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ACOG. Committee Opinion: Noninvasive Prenatal Testing for Fetal Aneuploidy. Available online through http://www.acog.org. Accessed January 19, 2013.
ACOG. New Prenatal Blood Test Promising. Press release, November 20, 2012. Available online at http://www.acog.org/About_ACOG/News_Room/News_Releases/2012/New_Prenatal_Blood_Test_Promising through http://www.acog.org. Accessed January 19, 2013.
ISPD Rapid Response Statement. Prenatal Detection of Down Syndrome using Massively Parallel Sequencing (MPS): a rapid response statement from a committee on behalf of the Board of the International Society for Prenatal Diagnosis, 24 October 2011. PDF available for download at http://www.ispdhome.org/public/news/2011/ISPD_RapidResponse_MPS_24Oct11.pdf through http://www.ispdhome.org. Accessed January 19, 2013.
March of Dimes. The Leading Edge of Medical Innovation: New Prenatal Genetic Tests Use Mom's Blood to Learn about Her Baby. Available online at http://www.marchofdimes.com/news/10929.html through http://www.marchofdimes.com. Accessed January 19, 2013.
March of Dimes. New Prenatal Blood Test. Available online at http://newsmomsneed.marchofdimes.com/?tag=maternal-blood-screening through http://newsmomsneed.marchofdimes.com. Accessed January 19, 2013.
National Society of Genetic Counselors (NSGC). Noninvasive prenatal testing/noninvasive prenatal diagnosis: the position of the National Society of Genetic Counselors. PDF available for download at http://www.nsgc.org/Portals/0/Advocacy/NSGC%20Noninvasive%20Prenatal%20Testing%204-17-2012.pdf through http://www.nsgc.org. Accessed January 22, 2013.
Communication with Dr. Lee Shulman, Director, Division of Genetics, Feinberg School of Medicine at Northwestern University. January 2013.
Biba, Erin. This Simple Blood Test Reveals Birth Defects - And the Future of Pregnancy. Available online at http://www.wired.com/wiredscience/2012/12/ff-prenatal-testing through http://www.wired.com. Accessed January 2013.
A new prenatal test for spotting genetic issues is less invasive, but it's pricey. Washington Post. November 26, 2012. Available online through http://www.washingtonpost.com. Accessed January 2013.
MayoClinic.com. First trimester screening: Results. Available online at http://www.mayoclinic.com/health/first-trimester-screening/MY00126/DSECTION=results through http://www.mayoclinic.com. Accessed January 2013.