During pregnancy, a small amount of genetic material from the placenta circulates in the mother’s bloodstream. This material is known as cell-free DNA (cfDNA). Fetal fraction describes the proportion of this placental cfDNA compared to the mother’s own cfDNA. It is a measurement of how much of the baby’s genetic material can be detected in a maternal blood sample.
Imagine the mother’s blood is a puzzle made of her DNA pieces, with a few pieces from the placental DNA mixed in. The fetal fraction tells us the percentage of these “different” puzzle pieces. This small sample provides a window into the genetic makeup of the developing fetus.
The Role of Fetal Fraction in Prenatal Screening
Fetal fraction is an important measurement for non-invasive prenatal testing (NIPT). This test analyzes cell-free DNA from a maternal blood sample to screen for specific chromosomal conditions in the fetus, such as Down syndrome. Because the test relies on detecting these small fragments of placental DNA, there must be enough of it present to ensure the results are reliable. If the amount of fetal DNA is too low, it is difficult to distinguish from the mother’s genetic background.
For a laboratory to provide a result, the fetal fraction needs to be above a certain threshold. This minimum requirement is often around 4%, although it can vary between testing laboratories. A higher fetal fraction provides a clearer distinction between a sample with a chromosomal abnormality and one without, increasing the confidence in the test’s outcome.
It is important to understand that NIPT is a screening test, not a diagnostic one. A screening test assesses the chance of a condition being present, while a diagnostic test, like amniocentesis, provides a more definitive answer but carries a small risk of complications.
Factors That Influence Fetal Fraction Levels
Several biological factors can influence the percentage of fetal fraction in a pregnant person’s blood. One of the primary factors is gestational age. The amount of placental cfDNA increases as the pregnancy progresses, so a blood sample taken at 10 weeks of gestation will have a lower fetal fraction than one taken at 12 or 14 weeks.
Another primary factor is the mother’s body mass index (BMI). Individuals with a higher BMI have a larger overall blood volume. This increased volume can have a dilution effect on the placental cfDNA, resulting in a lower fetal fraction. This does not indicate a problem with the pregnancy, but is a consequence of the larger blood volume.
Other circumstances can also play a role. For instance, certain types of in vitro fertilization (IVF) may affect the measurement. Additionally, carrying twins or other multiples can influence the levels of cfDNA, although the exact impact can be complex.
Implications of a Low Fetal Fraction
Sometimes, a NIPT result will come back as “inconclusive” or “no-call.” The most common reason for this is a low fetal fraction, meaning there was not enough placental DNA in the sample to analyze confidently. This result is not an indication that there is something wrong with the fetus; it simply means the test could not be completed accurately.
When a test fails due to a low fetal fraction, the recommendation is to repeat the blood draw. This is often scheduled a week or two after the initial attempt to allow more time for the fetal fraction to increase with gestational age. In most cases, a second sample will yield a sufficient fetal fraction for a conclusive result.
If a second attempt still results in a low fetal fraction, a healthcare provider will discuss other options. These might include different types of screening tests or moving to diagnostic tests like amniocentesis. The conversation will be tailored to the individual’s specific circumstances and medical history.