Prenatal diagnosis of tetrasomy 21.

Amniocentesis and prenatal diagnosis were done for late maternal age and an abnormality consistent with Tetrasomy 21 (47,XX, + t(21;21] was found in every cell examined of the initial amniotic fluid. Clinical examination revealed a fetus with many of the signs of Down Syndrome and pathological examination revealed gross abnormalities of the internal structures. Follow-up tissues showed mosaic Tetrasomy 21.