SWEDISH SURVEY ON EXTRA STRUCTURALLY ABNORMAL CHROMOSOMES IN 39105 CONSECUTIVE PRENATAL DIAGNOSES - PREVALENCE AND CHARACTERIZATION BY FLUORESCENCE IN-SITU HYBRIDIZATION

During 7 years (1985-1992), 39 105 consecutive prenatal diagnoses (34 908 amniocenteses and 4197 chorionic villus samples) were made at the five largest clinical genetic laboratories in Sweden. Thirty-one cases of extra structurally abnormal chromosomes (ESACs) were found, giving a total prevalence of 0.8 per 1000. Twelve ESACs were inherited, 14 w ere de novo and in five the parental origin was unknown. This gives an estimated prevalence of 0.3-0.4 per 1000 for familial and 0.4-0.5 per 1000 for de novo ESACs. Retrospectively, the ESACs were characterized by fluorescence in situ hybridization (FISH). In nine cases, no mater ial was available for this analysis. In 21 of the remaining 22 cases, the chromosomal origin could be identified by FISH. Seventeen of these (81 per cent) were derived from the acrocentric chromosomes, of which 13 originated from chromosome 15 (62 per cent). The most common ESAC was the inv dup(15) (57 per cent). Two cases were derived from chromos ome 22, one from chromosome 14, and one from either chromosome 13 or c hromosome 21. The four remaining cases consisted to two i(18p)s and tw o small ring chromosomes derived from chromosomes 4 and 19, respective ly.