Abstract: Objective: To investigate the association of maternal age and environmental factors with the risk of delivering a child with Down's syndrome (DS).Methods: The peripheral blood lymphocytes of the mother of the children with DS were cultured to prepare human G-banding chromosome examples for karyotype analysis,and the association of the maternal age and environmental factors with the risk of delivering a child with DS was analyzed.Results: One hundred and twenty-six of the 145 mothers of children with DS (86.90%) were below the age of 35.All the mothers had contacted with harmful physical or chemical factors to different degree before pregnancy.Conclusions: The age of the mother delivering a child with DS tends to be younger,which is probably due to the harmful environmental factors.Popularizing prepotency knowledge and enlarging the scope of prenatal screening and prenatal diagnosis may decrease the incidence of DS.