what does fetal fraction mean?

TLDR: Fetal fraction (FF) is the percentage of cell-free fetal DNA in maternal blood, crucial for the accuracy of non-invasive prenatal testing (NIPT). It influences test reliability, risk assessment for chromosomal abnormalities, and can indicate the need for repeat testing.

Fetal fraction (FF) is defined as the percentage of cell-free fetal DNA (cfDNA) in the maternal bloodstream that originates from the fetus, specifically from the placenta. It is essential for the reliability and accuracy of non-invasive prenatal testing (NIPT), which screens for fetal aneuploidies like Down syndrome. A minimum fetal fraction is necessary to ensure accurate detection of chromosomal abnormalities, as low fetal fractions can lead to inconclusive results or false negatives. Additionally, FF can serve as a marker for assessing the risk of aneuploidy, with studies indicating that lower fetal fractions may correlate with increased risks.

Several factors influence fetal fraction, including gestational age, maternal weight, and pregnancy type. Fetal fraction typically increases with gestational age, peaks between 10 and 21 weeks, and is generally higher in twin pregnancies. Low fetal fraction can result in "no-call" results in NIPT, necessitating repeat testing, and is associated with a higher risk of fetal aneuploidy. Understanding fetal fraction is crucial for clinicians to provide effective prenatal care and counseling, as it plays a significant role in the accuracy and outcomes of prenatal testing.