This paper reported a pair of twin babies who were diagnosed as monochorionic (MC) twins (MC) in the first-trimester ultrasound scan after in vitro fertilization.Fluorescence in situ hybridization and chromosomal karyotype analysis of amniotic fluid that was randomly collected from one of the twins were performed in the second trimester,but no abnormality was detected.Chromosome karyotype analysis of peripheral blood of the twins at the age of two days and three years showed that both were chi 47,XY,+21/46,XX.And the single nucleotide polymorphism microarray analysis of oral mucosa cells of the twins at three years old showed that the boy was 47,XY,+21 and the girl was 46,XX.Therefore,we confirmed that they are MC dizygotic twins presenting with blood chimerism but no tissue chimerism.Simply the boy presented with facial features of Down syndrome.