Noninvasive prenatal testing (NIPT) using cell-free DNA (cfDNA) in maternal blood is being embraced by patients. The rapid evolution of such a test requires that health care providers stay up to date with developments in the field. After 10 weeks of gestation, approximately 10-15% of the total cfDNA in maternal plasma is of placental origin. Given that the cfDNA present in maternal plasma is a mixture of maternal and placental cfDNA, false-positives can occur, and may be due to a number of biologic phenomena including confined placental mosaicism, a vanishing twin, maternal mosaicism, copy number variants, or subclinical maternal cancer.
NIPT is considered an advanced screening test, and the standard recommendation is that an abnormal result should be followed by a diagnostic test. Many authors and professional organizations have suggested that the diagnostic test could be either chorionic villous sampling (CVS) or amniocentesis. I would like to bring to the attention of the readers of the American Journal of Obstetrics and Gynecology that CVS examines placental tissue, and a proportion of false-positive NIPT cases could be due to placental mosaicism. Since amniotic fluid cells are largely derived from the fetus, I would like to suggest that the optimal diagnostic test be based on the results of amniocentesis, rather than CVS, however the results of NIPT may be available at 11-13 weeks, but the amniocenteses may need to wait until 15-16 weeks.
References
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