Background: Noninvasive prenatal testing based on massively parallel sequencing (MPS) of cell-free DNA in maternal plasma has become rapidly integrated into clinical practice for detecting fetal chromosomal aneuploidy. We directly determined the fetal fraction (FF) from results obtained with MPS tag counting and examined the relationships of FF to such biological parameters as fetal karyotype and maternal demographics.
Methods: FF was determined from samples previously collected for the MELISSA (Maternal Blood Is Source to Accurately Diagnose Fetal Aneuploidy) study. Samples were resequenced, analyzed blindly, and aligned to the human genome (assembly hg19). FF was calculated in pregnancies with male or aneuploid fetuses by means of an equation that incorporated the ratio of the tags in these samples to those of a euploid training set.
Results: The mean (SD) FF from euploid male pregnancies was 0.126 (0.052) (n = 160). Weak but statistically significant correlations were found between FF and the maternal body mass index (r(2) = 0.18; P = 2.3 × 10(-8)) and between FF and gestational age (r(2) = 0.02; P = 0.047). No relationship with maternal ethnicity or age was observed. Mean FF values for trisomies 21 (n = 90), 18 (n = 38), and 13 (n = 16) and for monosomy X (n = 20) were 0.135 (0.051), 0.089 (0.039), 0.090 (0.062), and 0.106 (0.045), respectively.
Conclusions: MPS tag-count data can be used to determine FF directly and accurately. Compared with male euploid fetuses, the FF is higher in maternal plasma when the fetus has trisomy 21 and is lower when the fetus has trisomy 18, 13, or monosomy X. The different biologies of these aneuploidies have practical implications for the determination of cutoff values, which in turn will affect the diagnostic sensitivity and specificity of the test.