Mosaic and hypermodal cells in amniotic fluid cell cultures.

Karyotypic results from a series of 975 consecutive amniotic fluid specimens studied for prenatal diagnosis are reported. In only 32% of specimens were all cells analyzed euploid. Two thirds (63%) of the specimens studied were hypomodal in at least one cell; 25% of the studies were hypomodal in three or more cells. In contrast, only 3.5% of the studies were hypermoidal in one or more cells. Most frequently an additional chromosome 2 was present in these hypermodal cells. Cells with other than a diploid chromosome constitution may be mistakenly identified as evidence of mosaicism. This is especially likely when the involved chromosome is one associated with a clinically defined syndrome.