Constitutional chromosomal mosaicism is the result of postfertilization mitotic error, the mechanism of which is not fully understood. The distribution of mosaicism in the conceptus depends on the timing, cell lineage(s) involved, cell viability, and chromosome involved. The developmental consequences of mosaicism also are related to its meiotic or somatic type. Meiotic mosaicism often is associated with a more severely adverse effect on the conceptus (see trisomy zygote rescue) due to the presence of uniparental disomy in the embryo/fetus and/or to dysfunction of a trisomic placenta. As mosaicism can be tissue specific, the result of a normal karyotype in cultured lymphocytes does not exclude the presence of mosaicism elsewhere in the conceptus. Mosaicism can best be detected by a combination of traditional cytogenetic analysis with molecular cytogenetic techniques such as comparative genomic hybridization and fluorescence in situ hybridization.