Mosaicism detected in the cytogenetic analysis of chorionic villi or cultured amniocytes can present a difficult and at times impossible interpretative dilemma. The finding may be indicative of a generalized fetal mosaicism. However, in the majority of cases, the abnormal cell line is representative of either an in-vitro event or a chromosomal error that is restricted to the extra-embryonic tissues. Advances in laboratory medicine have provided insight into the determination of which cases of true mosaicism have the greatest risk of being clinically significant with regard to the fetal genotype and phenotype. Despite the presence of a normal fetal karyotype, certain chromosomes involved in confined placental mosaiciam may increase the risk of poor perinatal outcome or predispose the fetus or neonate to the adverse effects of genetic imprinting owing to the development of uniparental disomy.