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 weeks' gestation. Chromosome analysis of fetal blood, cultured chorionic villi and fetal fibroblasts confirmed the aberrant karyotype. The aberrant chromosome complement could not be detected in a short-term culture of chorionic villi (46,XX in 21 metaphases). These findings suggest that the aberrant karyotype was a postzygotic event. Fetal ultrasound was normal and the parents decided to terminate the pregnancy at 21 weeks' gestation. Autopsy revealed multiple abnormalities including facial dysmorphy, partial agenesis of the corpus callosum and an interatrial septal defect.