Fetal fraction is the percentage of a developing baby’s genetic material circulating in the pregnant person’s bloodstream. This measurement indicates the proportion of the baby’s DNA compared to the pregnant person’s own DNA within a blood sample. It is used in prenatal screenings to assess test reliability.
The Source of Fetal DNA in Maternal Blood
During pregnancy, the placenta, an organ that connects the baby to the pregnant person, naturally sheds small fragments of its DNA. These tiny pieces, known as cell-free DNA (cfDNA), enter the pregnant person’s bloodstream. Since the placenta generally shares the same genetic makeup as the developing baby, these fragments are considered representative of the fetal DNA.
These fetal DNA fragments can be detected as early as five to seven weeks of gestation and typically clear from the maternal bloodstream within hours after childbirth.
Interpreting Fetal Fraction Percentages
For many prenatal screenings, such as non-invasive prenatal testing (NIPT), a minimum fetal fraction is necessary to ensure accurate and reliable results. This threshold typically falls within the range of 2% to 4%, with some laboratories requiring at least 4% for a conclusive outcome.
A low or “no call” fetal fraction result means there was not enough fetal DNA in the sample to confidently perform the analysis. This does not suggest a problem with the baby’s health or development, but rather indicates that the test could not generate a definitive result for chromosomal conditions.
A higher fetal fraction generally allows for more accurate detection of chromosomal differences, improving the sensitivity and specificity of the screening. Conversely, a lower proportion of fetal DNA can increase the chance of an inconclusive result or, in rare instances, a false negative.
Factors That Affect Fetal Fraction Levels
Several biological factors can influence the amount of fetal DNA circulating in the pregnant person’s blood.
The gestational age of the pregnancy is a significant determinant; fetal fraction naturally increases as pregnancy progresses, making tests more reliable later in the first trimester or early second trimester. For instance, the average fetal fraction is around 4% to 6% at week 10, rising to over 10% by week 20.
Maternal body mass index (BMI) also plays a role, with a higher BMI often correlating with a lower fetal fraction. This is thought to be due to a dilution effect from increased maternal blood volume or increased maternal cell-free DNA.
Certain medications, like some blood thinners such as heparin or enoxaparin, have been associated with reduced fetal fraction.
Specific pregnancy conditions can also affect these levels. For instance, vanishing twin syndrome, where one twin is reabsorbed, can interfere with the measurement. Certain fetal chromosomal conditions, such as trisomy 13 or 18, may be associated with lower fetal fractions, while trisomy 21 (Down syndrome) may be associated with higher levels.
Next Steps for a Low Fetal Fraction Result
Receiving a low fetal fraction result can be concerning, but it is important to remember that it often just means the test could not be completed.
In such cases, the most common recommendation from healthcare providers is to have a blood redraw after a period of one to two weeks. This allows additional time for the fetal fraction to naturally increase in the bloodstream, improving the chances of a successful re-test.
Approximately half of repeat blood draws will yield a conclusive result. If a second test also results in a low fetal fraction, or if a redraw is not feasible, healthcare providers may discuss alternative screening options. These could include traditional prenatal screening tests like a quad screen, or diagnostic procedures such as amniocentesis or chorionic villus sampling, which provide definitive genetic information about the baby.