We wish to thank Dar et al for their comments, especially regarding the need for caution when using chorionic villus sampling (CVS) as follow up to abnormal noninvasive prenatal screening (NIPS). We agree that amniocentesis is, indeed, the better option than CVS for follow-up evaluation to NIPS. Because the “fetal” component of the cell-free DNA that is used in NIPS is actually trophoblast in origin like chorionic villi, aneuploidy suspected by that screening method is best confirmed by cytogenetic studies on amniotic fluid cells because chorionic villi may simply be mirroring the NIPS “false positives.” Confined placental mosaicism of the types that can result in a false-positive CVS cytogenetic result occurs in approximately 0.8% of pregnancies (309/52,673 pregnancies); a fraction of those would have a sufficiently high percentage of mosaicism to result in a positive NIPS result. In spite of the shortcoming of CVS as a method of determining the accuracy of NIPS, part of the intent of our article was to focus on the performance of NIPS from the viewpoint of a cytogenetics laboratory. In our experience, 32% of our NIPS follow-up diagnostic samples were CVS. This suggests that many patients who have early NIPS may not want to wait until 15 weeks gestation for clarification of a positive NIPS result by amniocentesis.
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