Hypermodal cells in amniotic fluid cultures: frequency, interpretation, and clinical significance

Specimens of amniotic fluid were studied to assess the frequency of hypermodal cells and their significance. Multiple cultures were initiated on each specimen, and thorough cytogenetic analysis was performed routinely. Of 1,000 specimens, 59 (5.9%) contained a total of 64 hypermodal cells. However, true fetal mosaicism was not detected. An additional structurally abnormal chromosome accounted for 32 of the 64 hypermodal cells, and, in each case, cytogenetic analysis offered an explanation that obviated clinical concern. In 17 of the 32 cases, hypermodality was only apparent, the result of a single chromosome broken into two parts to produce a spuriously elevated count; in the other 15 cases, the extra chromosome was an unidentified fragment. In the 32 other hypermodal cells, an additional morphologically normal chromosome was present; however, in each case, failure to detect similar aberrations in other cultures initiated from the same specimen minimized the possibility that such cells reflected true fetal mosaicism. Indeed, clinical outcome was normal in all cases. Thus, for clinical purposes, routine banding techniques and analysis of multiple cultures seem to be adequate to exclude fetal mosaicism; laborious in situ methodology seems to be unnecessary. Future cytogenetic efforts should emphasize processing of more specimens of amniotic fluid, rather than more detailed analysis of individual specimens.