Circulating Fetal Cell-Free DNA Fractions Differ in

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 biolog-ical 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. Sam-ples 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 th