OP05.03: Changes in fetal cell‐free DNA fraction between consecutive maternal plasma samples in individual pregnancies

The measurement of percent fetal cell-free DNA (cfDNA), or fetal fraction, is recognized as an essential quality metric in accurate fetal aneuploidy assessment. Previous studies in large clinical populations show an increase in fetal fraction with gestational age in aggregate. However, few studies have examined changes in fetal fraction in individual pregnancies across different time points. In this study, we determine changes in fetal fraction in the same patient over consecutive draws in singleton and twin pregnancy samples. A series of samples from a single commercial laboratory was queried to identify patients with two samples for cfDNA analysis for fetal trisomy (Harmony® prenatal test). Fetal fraction was determined by directed analysis at polymorphic loci. Maternal demographics, gestational age, draw date and test outcome were reviewed. 393 unique pregnancies were identified in which a second sample was submitted. In 388 singleton pregnancies, the median change in fetal fraction was a relative increase of 6.2% (range -59.5% to + 361.2%) with a median of 18 days between draws (range 1 to 140 days). Fetal fraction increased in 227 pregnancies (58.5%), decreased in 155 (40.0%) and remained unchanged in 6 (1.5%). The group with an increase in fetal fraction had more days between draws and a lower initial fetal fraction than the group with a decrease (median 21 versus 15 days, 9.3% versus 11.9%, respectively, p < 0.01). Median maternal weight and gestational age at initial draw were not statistically different between the two groups. In 5 twin pregnancies, the fetal fraction increased between draws (+ 2.3% to + 59%) over 9 to 61 days. Overall, fetal fraction increased between consecutive samples from twin and singleton pregnancies. However, 40% of singleton pregnancies studied had a lower fetal fraction in a second sample. Additional studies with more time points and multivariate methods would be worthwhile to investigate factors influencing fetal fraction in individual pregnancies. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.