Fetal Genetic Testing with Cell-Free DNA
If you have concerns about genetic conditions that may be present in your fetus, genetic testing can be offered or recommended during your prenatal care. Cell-free DNA-based screening, often referred to as cfDNA or non-invasive prenatal screening (NIPS) or non-invasive prenatal testing (NIPT) is a type of test that has been available since 2011. cfDNA is DNA that is released from the placenta and transferred into the mother’s bloodstream. Dr. Tamar Goldwaser, Medical Geneticist and OB-GYN at Carnegie Hill Imaging for Women and Maternal Fetal Medicine Associates, emphasizes the importance of offering this test to all pregnant women.
What is Prenatal Genetic Testing?
Prenatal genetic testing refers to a variety of tests that can either determine the likelihood of your child having a genetic disorder (prenatal screening) or diagnose a specific condition or disorder (prenatal diagnostic tests). Non-invasive prenatal genetic screening using cell-free DNA (cfDNA) in the mother’s blood is often done first before more invasive testing like chorionic villus sampling (CVS) or amniocentesis.
Different Types of Prenatal Genetic Testing
Two main types of testing can be done during the first trimester to screen for genetic abnormalities and chromosomal disorders caused by aneuploidy, the presence of extra chromosomes, or the lack of one or more chromosomes. These are the First Trimester Screen (FTS) and cfDNA-based screening.
FTS involves an ultrasound measurement of the fetus (called the nuchal translucency) between 11 and 13 weeks of pregnancy and a blood test to measure proteins and hormones in the mother’s blood. The blood work and ultrasound findings are used to calculate the likelihood that the fetus has an extra or missing chromosome in the FTS. cfDNA-based screening relies on a blood test only, to measure the circulating placental DNA found in the mother’s bloodstream. Dr. Goldwaser recommends cfDNA over the FTS when possible.
Benefits and Concerns of Ultrasound Testing
First-trimester screening (FTS) with ultrasound and biomarkers can detect abnormalities in the placenta or the growth of the fetus. It can also detect extra or missing chromosomes like Trisomy 21 (Down Syndrome). However, only 1 in 20 women (about 5%) who receive a positive result will actually have a child born with Down syndrome. The FTS has a higher false positive rate than the cfDNA-based screening.
Benefits and Concerns of cfDNA Testing
cfDNA can be performed as early as 9 weeks, as opposed to ultrasonic testing performed at 11-14 weeks, and it only has a 0.04% false positive rate. Additionally, it’s the only screening available for the detection of sex chromosome aneuploidies, microdeletion, and genome-wide changes. Microdeletion and genome-wide changes are less common chromosomal abnormalities but may also have a significant impact on the baby’s growth and development.
Common Genetic Disorders Detected by Cell-Free DNA
The American College of Medical Genetics and Genomics (ACMGG), the Society for Maternal Fetal Medicine (SMFM), and the American College of Obstetricians and Gynecologists (ACOG) all recommend cfDNA as the first choice of test to detect the following genetic diseases:
- Down Syndrome (Trisomy 21)
- Edward Syndrome (Trisomy 18)
- Patau Syndrome (Trisomy 13)
ACMGG recommends cfDNA screening for fetal sex chromosome aneuploidies like:
- Turner Syndrome: affects women with symptoms like short height, delayed puberty, infertility, learning disabilities
- Jacobs Syndrome: affects men with symptoms like tall stature, risk of disabilities, autism, or ADHD
- Klinefelter Syndrome affects men with symptoms like gynecomastia, osteoporosis, infertility, and learning disabilities
Despite the ability of cDNA to detect many possible abnormalities with a detection rate of 98.8%, it’s important to remember that this is a screening test. If you receive a positive test, invasive diagnostic testing will be needed to determine with certainty whether a child has a genetic disorder.
What to Do if You Get a Positive Test?
If you get a positive test, this does not mean your child is guaranteed to have a genetic disorder. The positive predictive value of cfDNA (how likely it is that the positive result is correct) is as follows:
- Down Syndrome: PPV 91.8%
- Edward Syndrome: PPV 65.8%
- Patau Syndrome: PPV 37.2%
If you receive a positive test, your geneticist will recommend genetic counseling services and diagnostic testing.
Genetic Testing at MFM
Dr. Tamar Goldwaser is a board-certified OBGYN and our expert Medical Geneticist. At our office, you can expect the newest, cutting-edge medical care for the best possible treatment plan. If you need prenatal screening tests in New York City or wish to speak to a maternal-fetal medicine specialist, we encourage you to schedule an appointment today.
Maternal Fetal Medicine blogs are intended for educational purposes only and do not replace certified professional care. Medical conditions vary and change frequently. Please ask your doctor any questions you may have regarding your condition to receive a proper diagnosis or risk analysis. Thank you!