Chorionic villus sampling fails to confirm mosaic trisomy 21 fetus after positive cell‐free DNA

What's already known about this topic? Prenatal screening for certain fetal chromosomal conditions can be performed using cell‐free DNA (cfDNA) in maternal plasma. cfDNA is thought to originate primarily from apoptosis of placental cytotrophoblast and syncytiotrophoblast cells; therefore, a false negative result may not be representative of the fetal karyotype. A positive cfDNA screen requires confirmation with diagnostic testing, such as chorionic villus sampling (CVS) or amniocentesis. What does this study add? This case illustrates detection of fetal and placental trisomy 21 mosaicism by cfDNA that was not confirmed with CVS. Patients for whom cfDNA is appropriate should be offered prior genetic counseling. Those who choose diagnostic testing via CVS should be counseled on the possibility of true fetal or confined placental mosaicism. Amniocentesis preferentially should be offered, as CVS represents a similar cell origin as those from which the cfDNA results are derived.