Background: Double aneuploidy is the phenomenon where two aneuploidies co-exist in the same individual, usually involving one autosomal chromosome and one sex chromosome. Double autosomal aneuploidy is rare and usually results in spontaneous abortions. There are only six published case reports of liveborn with trisomy 18 and trisomy 21 and all of which involve mosaicism. Case Presentation: This case report documents an infant born at 35 weeks with phenotypic features of Down Syndrome. However, cytogenetic analysis showed a mosaic of both trisomy 18 and 21. The patient initially had patent ductus arteriosus requiring operative closure and has congenital hypothyroidism. Conclusion: We describe five possible pathways leading to this phenomenon, including error during meiosis, meiotic nondisjunction, mitotic nondisjunction, sequential mitotic segregation and chimerism. This case report reiterates the significance of traditional cytogenetic analysis in children with features of Down Syndrome to detect any further abnormalities.