A non‐mosaic trisomy 20 was discovered in all cells in two separate cultures from an age‐related genetic amniocentesis. Karyotypes of cells obtained via amniocentésis at the time of termination and of cells cultured from the placenta gave the same unambiguous results. However, the fetus, under macro‐ and microscopic analysis, showed only two minor anomalies: left simian crease and low‐set ears. These findings are more suggestive of a normal or at most mosaic trisomy 20 state. The significance of this finding for prenatal diagnosis is discussed.