Source: iStock/Aldo Murillo
Source: iStock/Aldo Murillo

Cell-free DNA from the maternal blood of a pregnant woman can throw clinicians a curve. Such DNA can indicate whether the genome of a developing fetus contains an abnormal number of chromosomes. Or it may simply indicate that the pregnant woman’s genome contains copy number variants (CNVs). If the latter possibility is not taken into account, a prenatal screen could yield a false-positive result. For example, the screen could overstate the odds that a developing fetus is at risk of certain genetic conditions, such as a chromosome trisomy.

The newer prenatal screens that rely on cell-free DNA have shown high sensitivity and specificity. Yet these tests have limited positive predictive value because the overall incidence of aneuploidy is low. As these prenatal screens become more common, false-positive results may become more worrisome.

This possibility was explored by researchers at the University of Washington, Fred Hutchinson Cancer Research Institute, and the Howard Hughes Medical Institute. These researchers investigated four pregnancies with discordant prenatal test results.

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