Although trisomy 16 is commonly detected in spontaneous abortions and accounts for over 30% of cases of autosomal trisomy detected after spontaneous abortion, trisomy 16 mosaicism is rarely detected by amniocentesis in the second trimester. Here, we report a case of level III trisomy 16 mosaicism (47,XX,+16[8]/46,XX[31]) diagnosed by cytogenetic analysis of independently cultured amniotic fluid cells. The female baby was delivered at full term with low birth weight and intrauterine growth retardation, and interestingly, her karyotype was normal (46,XX). Given the difficulty in predicting the outcomes of fetuses with this mosaicism, it is recommended to inform the possibility of mosaicisms including this trisomy 16 mosaicism during prenatal genetic diagnosis and genetic counseling for parents.