Background: The intent of noninvasive prenatal testing is to screen for fetal aneuploidies. The assumption is that overrepresented and underrepresented chromosomes are of fetal origin. However, this is not always the case.
Cases: We report three cases in which maternal sex chromosome aneuploidy (confirmed by karyotype), two cases of which were previously unknown, resulted in false-positive results. In each, results were positive for fetal aneuploidy, but neonatal karyotypes confirmed normal karyotype.
Conclusion: Noninvasive prenatal testing assesses the proportion of chromosomes 21, 18, 13, and sex chromosomes in maternal circulation. Intrinsic to the analysis is that the underrepresentations and overrepresentations are of fetal origin. We present three cases in which this assumption is not valid. We suggest that maternal sex chromosome aneuploidy be considered when results suggest fetal sex chromosome aneuploidies.