Aim: To investigate parental origins and cell stage errors of a double nondisjunction in a fetus. Method: For the determination of the most common chromosome anomalies, quantitative fluorescent polymerase chain reaction method using short tandem repeat (STR) DNA markers was applied to a fetus with abnormal ultrasonographic findings. Parental origin and cell stage errors of the trisomies were inferred by comparing the inherited STR alleles. Conventional cytogenetic technique was also applied for the confirmation of the aneuploidies. Results: A double nondisjunction including chromosomes 21 and X (48, XXX,+21) was detected prenatally in the fetus. The origin of both chromosomes was maternal, and the errors were in meiosis I for 21 and meiosis II for X. Molecular results were concordant with cytogenetic results. Conclusion: Molecular techniques could be useful for the pre- and postnatal diagnosis of the common aneuploidies and determining its parental origin. This kind of study will improve knowledge about the mechanisms of nondisjunction and enable appropriate and rapid genetic counseling.