NIPT is non-definitive testing to estimate the possibility that fetuses have trisomy 21, trisomy 18, or trisomy 13. However, in NIPT-positive and indeterminate cases, rare chromosomal disease may become apparent, requiring advanced genetic considerations and counseling skills. We experienced two such cases, a trisomy 21 mosaicism case triggered by NIPT-positive status and 18q deletion syndrome triggered by NIPT-indeterminate status. These cases have two clinical implications for NIPT. First, it was revealed that trisomy mosaicism might be found in NIPT-positive cases that have lower Z-Scores than those inferred from the fraction of fetal cfDNA in the case of standard trisomy. Second, it is possible that microdeletion syndrome could be the reason for an indeterminate NIPT result. Today's genetic counseling requires more expertise in ethics and communication as well as genetic science because NIPT can lead to totally unexpected results.
Keywords: 18q-deletion syndrome; NIPT; genetic counseling; massively parallel sequencing; trisomy 21 mosaicism.