Prenatal diagnosis of a de novo supernumerary marker chromosome originating from chromosome 16: A 37 year old pregnant woman was referred for amniocentesis at 18 weeks of gestation due to advanced maternal age and abnormal serum biochemistry. A nonsatellited, monocentric marker chromosome was observed with a frequency of 57% in cultured amniocytes. Parental karyotypes were normal. The marker chromosome was found to be derived from chromosome 16 by FISH using CEP16 and WCP16 probes. Marker chromosomes were not painted with M-FISH probe mixture, indicating an exclusively heterochromatin nature. CGH analysis using genomic DNA isolated from uncultured amniocytes also supported the M-FISH results. Genetic counseling was given to parents and the family decided to continue the pregnancy to term. The baby was born at 36 weeks of gestation without any dysmorphic features. Follow-up at 7 months of age revealed no developmental abnormalities.