Cell free DNA is primarily derived from the placenta
Sensitivity for trisomy 21, 18, 13 and the sex chromosomes is high, but the positive predictive value is lower and depends on maternal age
Accuracy of cell free DNA is limited in multiples
A diagnostic testing is recommended after positive cell free DNA screening
Options include chorionic villus sampling or amniocentesis
Amniocentesis usually done after 16 weeks, Chorionic villus sampling is done between 11-14 weeks
Serum analytes do not offer any additional benefit after cell free DNA
Repeat cell free DNA screening is not recommended
Paternal karyotypes are recommended only if diagnostic testing (amniocentesis or CVS) show a translocation type of trisomy which has a risk of recurrence

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