In the practice of clinical genetics chromosomal aneuploidy in both mosaic and nonmosaic forms has long been recognized as a cause of abnormal prenatal and postnatal development. Traditionally, cytogenetic analysis of cultured lymphocytes has been used as a standard test for detection of constitutional aneuploidies. As lymphocytes represent only one lineage, chromosomal mosaicism expressed in other tissues often remains undetected. The purpose of this study was to assess the utilization of molecular cytogenetic analysis for detection of chromosomal aneuploidy in placental tissues. Using placentas from 100 pregnancies with viable nonmalformed livebirths, both trophoblast and chorionic stroma were analyzed using comparative genomic hybridization (CGH). In all cases with an indication of chromosomal imbalance by CGH, fluorescence in situ hybridization (FISH) analysis was performed to confirm the presence of aneuploidy. To differentiate between constitutional aneuploidy and confined placental mosaicism (CPM), amniotic membrane was analyzed by CGH and FISH techniques. Our results demonstrated five placentas with CPM for chromosomes 2, 4, 12, 13, and 18, respectively, and two constitutional nonmosaic aneuploidies (47,XXX and 47,XXY). Molecular cytogenetic studies of human placental tissues enables easy analysis of both embryonic (amnion) and extraembryonic (chorion) cell lineages. Detection at birth of chromosomal defects affecting intrauterine placental and fetal development is important because these chromosomal defects may continue to have an influence on postnatal development.