Mosaicism or pseudomosaicism: the problem of hypermodal cells in amniotic fluid cell culture.

A series of 2029 consecutive amniotic fluid specimens studied for prenatal genetic diagnosis were reviewed and reassessed so as to evaluate the frequency and clinical significance of hypermodal cells in amniotic fluid cell cultures. Hypermodal cells were defined as those with more than 46 chromosomes, and were characterized by an additional structurally normal or structurally abnormal chromosome. Of 2029 specimens, 47 (2.31 per cent) contained a total of 167 hypermodal cells. True fetal mosaicism was detected in three cases (0.14 per cent). All had hypermodal cells in more than one culture flask or colony which contained the same aberrant chromosome complement. In all but one case the babies were normal when only one cell was hypermodal, or when several cells were hypermodal but present in only one colony or one culture vessel. One case had an extra No. 20 chromosome in one cell. Although the child had multiple anomalies, they were not characteristic of trisomy 20, and subsequent chromosomal study on the baby postnatally revealed a 46,XX karyotype. The in situ coverslip technique is recommended as the preferred method for prenatal diagnosis, and it is useful as an aid in differentiating true mosaicism from pseudomosaicism.