Objective:To analyze the genetic features of homologous Robertsonian translocation trisomy 21.Methods:This retrospective analysis involved 12 pedigrees in which singleton fetuses were prenatally diagnosed with homologous Robertsonian translocation trisomy 21 [46,XX/XY,+21,der(21;21)(q10;q10)] at the Maternal and Child Health Hospital of Guangxi Zhuang Autonomous Region from January 2012 to January 2023. Moreover, karyotype analysis results of the parental peripheral blood were obtained. The prenatal diagnosis results and genetic features in the 12 pedigrees were summarized using descriptive statistical analysis.Results:Among the 12 pedigrees, eight cases were de novo and the other four were maternally inherited. Three mothers in the four inherited cases had homologous Robertsonian translocation trisomy 21 and the other one was a homologous Robertsonian translocation carrier. The karyotypes of the four fathers were all normal. There were three families with multiple children, two of the couples with normal karyotypes had normal children, and the other couple had a child with homologous Robertsonian translocation trisomy 21 that was inherited from the mother with the same type of trisomy 21. Non-invasive prenatal testing was performed in two pedigrees during this pregnancy and the results showed that one case was at low risk and one was at high risk of trisomy 21. Further testing of the placenta after labor induction confirmed the low-risk case with low proportion of mosaic trisomy 21 (the proportion was 21% on the maternal side of the placenta and 9% on the fetal side). Conclusions:Most cases of homologous Robertsonian translocation trisomy 21 are de nove and few are inherited. Parents of probands with homologous Robertsonian translocation trisomy 21 should be routinely advised to undergo peripheral blood chromosome examination to find out whether they are carriers of homologous Robertsonian translocation.