Cell-free fetal DNA (cffDNA) testing is a non-invasive prenatal screening (NIPS) method that offers expectant parents insights into the likelihood of certain chromosomal conditions in their developing baby. It provides a screening assessment without posing direct risks to the fetus, helping guide discussions and decisions regarding pregnancy management.
Understanding Cell-Free Fetal DNA Testing
Cell-free fetal DNA testing relies on small fragments of fetal DNA circulating in the mother’s bloodstream. These fragments originate from the placenta. While maternal DNA fragments are typically longer, cffDNA fragments are considerably smaller, which helps in distinguishing them.
A healthcare provider collects a blood sample from the pregnant individual. This sample contains a mixture of the mother’s DNA and cell-free fetal DNA. Laboratories then analyze these fragments to assess the relative amounts from specific chromosomes.
The test can be performed starting at 10 weeks of pregnancy, when a sufficient amount of fetal DNA, known as the fetal fraction, needs to be present in the mother’s blood for accurate analysis. Generally, a fetal fraction above 4% is considered adequate for reliable results. Results are usually available within approximately two weeks of the blood draw.
Conditions Screened For
Cell-free fetal DNA testing primarily screens for common chromosomal abnormalities known as aneuploidies, which involve an abnormal number of chromosomes. The most frequently screened conditions include Trisomy 21 (Down syndrome), Trisomy 18 (Edwards syndrome), and Trisomy 13 (Patau syndrome).
Trisomy 18, or Edwards syndrome, and Trisomy 13, or Patau syndrome, are less common but are often associated with severe birth defects and a significantly reduced lifespan. Many pregnancies affected by Trisomy 18 or Trisomy 13 may result in pregnancy loss. The test looks for an extra copy of these specific chromosomes.
Beyond these common trisomies, many tests also screen for sex chromosome aneuploidies. These conditions involve an abnormal number of X or Y chromosomes. Examples include Turner syndrome (monosomy X), Klinefelter syndrome (XXY), Triple X syndrome (XXX), and Jacobs syndrome (XYY). Some tests may also screen for microdeletions or microduplications, which are smaller missing or duplicated segments of chromosomes.
Interpreting Test Results
Cell-free fetal DNA testing provides results typically categorized as “low risk” or “high risk.” A “low risk” result indicates that the fetus has a very low probability of having the specific chromosomal conditions screened for, suggesting that the likelihood is below a certain established threshold. While reassuring, a low-risk result does not entirely eliminate the possibility of any genetic condition, as the test does not screen for all possible abnormalities.
Conversely, a “high risk” result means there is an increased likelihood or higher probability that the fetus has a chromosomal abnormality. It is important to understand that cell-free fetal DNA testing is a screening test, not a diagnostic one. This means it estimates the chance of a condition being present but does not provide a definitive diagnosis.
For a high-risk result, follow-up diagnostic testing is recommended to confirm the presence or absence of a genetic condition. These diagnostic tests are invasive procedures that analyze fetal cells directly. Chorionic villus sampling (CVS) involves collecting tissue from the placenta, typically performed between 10 and 13 weeks of pregnancy. Amniocentesis involves taking a sample of the amniotic fluid surrounding the baby, usually done between 15 and 20 weeks of pregnancy. These diagnostic tests provide a definitive answer regarding the baby’s chromosomes.
Important Considerations
Cell-free fetal DNA testing offers high accuracy for common trisomies, with detection rates exceeding 99% for Down syndrome. The test also has a high specificity, meaning it is effective at correctly identifying cases where a chromosomal abnormality is not present, which helps reduce false positive results compared to traditional screening methods. However, it is important to acknowledge that false positives and false negatives can still occur.
A false positive result means the test indicates a high risk, but the fetus is actually unaffected. This can happen due to factors such as placental mosaicism, where the placenta contains chromosomal abnormalities not present in the fetus, or due to maternal chromosomal variations. A false negative result, where the test indicates low risk but a condition is present, is less common but still possible.
Cell-free fetal DNA testing does not screen for all possible genetic conditions or structural birth defects, such as neural tube defects. Therefore, other routine prenatal screenings and ultrasounds remain important components of comprehensive prenatal care. The cost of cell-free fetal DNA testing can vary, ranging from approximately $100 to over $2,000 in the U.S., with insurance coverage often depending on individual plans and whether the pregnancy is considered high-risk.