We report a rare case of mosaic ring chromosome 22 duplication/deletion in a fetus for whom karyotype analysis was required because of an abnormal finding in the maternal serum screening test and a choroid plexus cyst detected on prenatal ultrasound. Additional prenatal study of the amniotic fluid by fluorescence in situ hybridization was performed and the terminal 22q13.3 deletion was detected on ring chromosome. The final karyotype was 45,XX,-2 / 46,XX,r(22)(p11q13.2) / 46,XX,idicr(22)(p11q13.2;p11q13.2)dn.ishder(22)(N25+, ARSA-, ter-). The pegnancy was terminated. Cytogenetic analysis of the intracardiac blood also revealed ring 22 mosaicism with only one metaphase spread with idicr(22) as the unstable isodicentric rings are subsequently lost from most cells. We discuss the prenatal diagnosis of this rare condition.