Detecting Mosaicism of Monosomy X Using FISH in Prenatal Samples: Post High Risk NIPT

Abstract

Noninvasive prenatal testing (NIPT) is a highly specific and sensitive aneuploidy screening method with low false positive results. Sex chromosome aneuploidy (SCA) is not picked up in prenatal ultrasounds, as they may not have antenatally identifiable features, except for hydrops in monosomy X cases. Women with high risk NIPT results for SCAs are recommended to go for invasive prenatal diagnosis for confirmation by diagnostic tests like chromosome microarray, karyotyping, and/or fluorescence in situ hybridization (FISH). We present two cases that showed a high risk for monosomy X on NIPT. Chromosomal microarray was negative for SCA. Further, FISH was done to confirm the results and confirm the presence of low level mosaicism for monosomy X. FISH proves to be the test of choice to detect low level mosaicism in high risk NIPT cases with high positive predictive values.