Placental mesenchymal dysplasia (PMD) is a distinct placental disorder that may coexist with a normal fetus. In one-third of cases, the fetus exhibits Beckwith-Wiedemann Syndrome (BWS). In the present study, we report a case of PMD changes associated with an unusual genetic constitution. Pathological examination showed an enlarged placenta with a mixture of normal but also numerous clusters of grape-like fluid-filled vesicles confined to the stem villi without trophoblast proliferation. Some stem villi contained many large vessels filled by partially organized thrombi consistent with PMD. The fetus presented an enlarged liver and cytomegaly in the adrenal glands, hyperplastic islets of Langerhans in the pancreas, and some microcysts with cuboidal epithelium in the kidneys. These findings suggest the Beckwith-Wiedemann syndrome phenotype. DNA genetic markers showed three alleles for three independent markers and two alleles for the 12 others. Fluorescent in situ hybridization (FISH) demonstrated that villous trophoblast and fetal tissues are diploid. The haploid paternal complement found in the androgenetic cells was different from that found in biparental cells, suggesting a double fertilization event. Preferential distribution of the androgenetic cells into the placenta explains the predominance of molar villi with an apparently normal fetus. This represents a well-documented case of androgenic and biparental mixture of cell types in both fetal and placental tissues.