Non-Invasive Prenatal Testing (NIPT) has become a common screening option during pregnancy, providing insights into the genetic health of the developing baby. This test analyzes small fragments of DNA found in the pregnant person’s bloodstream. A key aspect of NIPT, and a factor that influences its reliability, is the fetal fraction.
Understanding Fetal Fraction
Fetal fraction refers to the percentage of cell-free DNA (cfDNA) in a pregnant person’s blood that originates from the placenta, representing the fetus. Both maternal and placental cells release cfDNA into the bloodstream, but the “fetal” DNA analyzed in NIPT is primarily placental in origin. A sufficient amount of fetal cfDNA is necessary for reliable results. Most laboratories typically require a minimum fetal fraction of 2% to 4% for accurate analysis. The average fetal fraction is usually between 4% and 15% between 10 and 20 weeks of gestation, and it generally increases as pregnancy progresses.
Is High Fetal Fraction a Concern?
A high fetal fraction is generally not a cause for concern and is a positive indicator for NIPT reliability. A higher fetal fraction means there is more fetal DNA available for analysis, which can lead to more confident and accurate test results. Higher levels can be attributed to several common physiological reasons, including advanced gestational age, increased maternal weight, multiple pregnancies, or a larger placental size.
While very rarely, extremely high fetal fraction values have been investigated for potential associations with certain conditions like abnormally invasive placenta or spontaneous preterm birth. However, these associations are uncommon and typically identified through other clinical markers, not solely based on a high fetal fraction. For the vast majority of pregnancies, a high fetal fraction is considered benign.
Impact on NIPT Accuracy
A sufficient fetal fraction ensures enough fetal DNA is present to accurately detect or rule out chromosomal conditions. This helps prevent “no-call” or inconclusive results, which can occur when the fetal fraction is too low.
A higher fetal fraction improves the sensitivity and specificity of the NIPT. This means the test is better equipped to correctly identify chromosomal abnormalities when present and indicate their absence when not. In contrast, a low fetal fraction can increase the chance of an inconclusive result or, in rare cases, a false negative.
Next Steps and Clinical Guidance
If you receive NIPT results with a high fetal fraction, discuss these findings with your healthcare provider, such as your doctor or a genetic counselor. They can interpret the results within the context of your complete medical history and any other screening tests you have undergone.
Typically, if a high fetal fraction is the only notable finding and NIPT results indicate a low risk for chromosomal conditions, no further action is usually required. Your healthcare provider will offer personalized guidance and reassurance based on your unique circumstances.