Prenatal diagnosis and fetopathological findings in a fetus with ring chromosome 18

We report on a fetus with ring chromosome 18 and monosomy 18 mosaicism [mos 45,XX,-18/46,XX,r(18)] detected in cultured amniotic fluid cells at 15 wee ks' gestation. Chromosome analysis of fetal blood, cultured chorionic villi and fetal fibroblasts confirmed the aberrant karyotype. The aberrant chrom osome complement could not be detected in a short-term culture of chorionic villi (46,XX in 21 metaphases). These findings suggest that the aberrant k aryotype was a postzygotic event. Fetal ultrasound was normal and the paren ts decided to terminate the pregnancy at 21 weeks' gestation. Autopsy revea led multiple abnormalities including facial dysmorphy, partial agenesis of the corpus callosum and an interatrial septal defect.