A 10‐year survey, 1980–1990, of prenatally diagnosed small supernumerary marker chromosomes, identified by fish analysis. Outcome and follow‐up of 14 cases diagnosed in a series of 12 699 prenatal samples

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Abstract
A cytogenetic survey and follow‐up studies were made of 14 cases with supernumerary marker chromosomes, identified among 12 699 prenatal samples, investigated at our institution over a 10‐year period from 1980 to 1990. FISH (fluorescence in situ hybridization) techniques were employed to identify the chromosomal origin of the marker chromosomes. Five cases were familial, all derived from acrocentric chromosomes, and all without apparent phenotypic effects in the children. Nine cases represented de novo aberrations. In two cases (one with a marker from chromosome 14 or 22, the other with a ring‐like marker derived from chromosome 17), the pregnancies continued and apparently normal babies were delivered at term, but the child with a marker derived from chromosome 17 showed slight psychomotor retardation at 2 years of age. All other pregnancies with de novo markers were terminated. In three cases, significant abnormalities were found at autopsy. One of these had an isochromosome 12p and the phenotype was consistent with Pallister‐Killian syndrome. In conclusion, marker chromosome identification, as well as clinical follow‐up, is essential for the purpose of improving genetic counselling.