Cell-free DNA (cfDNA) screening is a noninvasive prenatal test that assesses the chance of a developing fetus having certain chromosomal conditions. This screening involves a simple blood draw from the pregnant person, making it a low-risk option. The test provides an estimate of risk rather than a definitive diagnosis.
The Science Behind cfDNA Screening
During pregnancy, small fragments of DNA from the placenta circulate in the pregnant person’s bloodstream. These fragments, known as cell-free DNA, largely represent the fetus’s genetic makeup. The cfDNA test works by analyzing these circulating DNA pieces to detect potential chromosomal abnormalities. It compares the proportions of DNA from different chromosomes to identify any imbalances.
A sufficient amount of fetal DNA, referred to as the “fetal fraction,” is necessary in the maternal blood for an accurate analysis. This fraction increases as the pregnancy progresses. The test is performed after 10 weeks of gestation. If the fetal fraction is too low, the test may not yield a result, requiring a redraw.
Conditions Screened by the Test
The cfDNA test screens for common trisomies, which are conditions caused by an extra copy of a chromosome. Trisomy 21, also known as Down syndrome, involves an extra copy of chromosome 21. Individuals with Down syndrome often experience developmental delays and may have distinct facial features, along with potential heart or organ issues.
The test also screens for Trisomy 18 (Edwards syndrome) and Trisomy 13 (Patau syndrome). These conditions are considered more severe than Down syndrome, often leading to multiple birth defects and a significantly reduced life expectancy.
Beyond these autosomal trisomies, cfDNA screening can identify sex chromosome aneuploidies, which involve variations in the number of X or Y chromosomes. Examples include Turner syndrome (a single X chromosome) and Klinefelter syndrome (XXY). The test can also reliably determine the fetal sex. Some expanded versions of the test may also screen for microdeletions, which are small missing pieces of a chromosome, though these are less common in standard screenings.
Interpreting Test Outcomes
Results from cfDNA screening are reported as either “low-risk” or “high-risk,” rather than a definitive “positive” or “negative.” A low-risk result indicates a small chance of the screened conditions being present, but it does not rule out their possibility. Conversely, a high-risk result suggests an increased likelihood of a condition.
cfDNA is a screening test, not a diagnostic one; it estimates the probability of a condition but cannot definitively state whether the fetus has it. In some instances, a high-risk result might occur even when the fetus is unaffected, known as a false positive. Similarly, a rare false negative can happen where the test indicates a low risk, but the condition is actually present.
Procedures Following a High-Risk Result
Receiving a high-risk cfDNA result can be concerning, but it marks the beginning of a clearer pathway for further evaluation. Genetic counseling is recommended to help parents understand the specific implications of the result and discuss available options. A genetic counselor can provide detailed information about the condition, its potential impact, and the accuracy of the screening test.
To obtain a conclusive diagnosis, definitive diagnostic testing is usually offered. The two primary procedures for this are chorionic villus sampling (CVS) and amniocentesis. CVS involves taking a small tissue sample from the placenta, usually performed in the first trimester, typically between 10 and 13 weeks of gestation.
Amniocentesis involves collecting a small amount of amniotic fluid surrounding the fetus, usually performed in the second trimester, generally between 15 and 20 weeks of pregnancy. Both CVS and amniocentesis directly analyze fetal cells, providing a definitive answer regarding the presence or absence of the chromosomal condition indicated by the cfDNA screening. Parents can then make informed decisions about their pregnancy based on this confirmed information.