Copy-Number Variation and False Positive Prenatal Aneuploidy Screening Results

False positive results for trisomy 18 from DNA-based noninvasive prenatal screening were caused, in two of the four patients who were evaluated, by maternal copy-number variation in chromosome 18. Methods of noninvasive prenatal screening 1 have advanced rapidly in clinical practice, with aneuploidy screening based on analysis of circulating cfDNA now routinely offered to women with high-risk pregnancies. Owing to the high reported accuracy of these screening tests, 2 , 3 attention has shifted to low-risk cohorts, in which the reduced incidence of aneuploidy may limit the positive predictive value of noninvasive prenatal screening. 4 A recent prospective analysis of cfDNA-based noninvasive prenatal screening in 1914 low-risk pregnancies showed false positive rates of 0.3%, 0.2%, and 0.1% for trisomies 21, 18, and 13, respectively — rates that were lower than those observed with . . .