Technical and biological aspects of pseudomosaicism and mosaicism.

In 471 amniotic fluid cell cultures, single abnormal cells were found to be randomly distributed. The expected number of pseudomosaicisms for aneuploidy due to randomly distributed cells was 3.9, and the observed number was 4. Expected number of mosaicism for aneuploidy was 0.6, and none was observed. The findings indicated that pseudomosaicisms as well as mosaicisms may be diagnosed from randomly distributed single abnormal cells. The probability of erroneously confirming mosaicism increased with the number of cells analysed in the second culture.