Grati FR, et al. – Here, authors aim is to predict the i) false positive/negative rates (FPR/FNR) of cell–free DNA (cfDNA) testing consequent to feto–placental mosaicism for any sex chromosome anomaly (SCA) ii) positive (PPV) and negative predictive values (NPV) of a high– and low–risk cfDNA result for any SCA. Mosaicism is a major issue for SCA cfDNA testing, and prenatal confirmation, preferentially with amniocentesis if there are no ultrasound anomalies, remains important in counseling. As positive predictive values (PPV) varies based on the presence of an ultrasound anomaly, skilled assessment is critical.