Objective To describe the molecular and cytogenetic characterization of two different prenatal cases of androgenetic/biparental mosaicism and review the different possible mechanisms of origin in each case. Design Case study and literature review. Setting Tertiary medical center (prenatal diagnosis unit). Patient(s) A 26-year-old pregnant woman referred for suspected partial mole placenta and a 33-year-old pregnant woman referred for polyhydramnios and fetal malformations. Intervention(s) Ultrasound examination, prenatal invasive procedures, molecular and cytogenetic analysis, physical and pathologic evaluation, and genetic counseling. Main Outcome Measure(s) Cytogenetic analysis, fluorescent in situ hybridization, and quantitative fluorescence polymerase chain reaction (QF-PCR) analysis. Result(s) The finding of a normal karyotype together with a triploidy-like QF-PCR profile led to the diagnosis of two cases of androgenetic (genome-wide paternal uniparental disomy)/biparental mosaicism. The first case showed placental mesenchymal dysplasia and a normal fetus, and the second one presented a fetus showing Beckwith-Wiedemann syndrome features and an apparently normal placenta. Conclusion(s) These cases highlight the wide range of possible clinical presentations of androgenetic/biparental mosaicism, the variety of mechanisms of their origin, and the importance of the combination of molecular and cytogenetic analysis to achieve an accurate diagnosis and provide reproductive counseling.