To explore the influencing factors of major birth defects.The children of 0 -3 years and their mother from six different districts were surveyed from November 2009 to December 2009.The incidence rate was 134.27/10 000 (493/36 716)in 6 counties. And the first five birth defects were as follows:congenital heart disease,neural tube defects,cheilopalatognathus,polydactylia and hypophrenia.The risks and protective factors of the first 5 birth defects were different.Anamnesis of birth defects and defective children from relatives,multiple pregnancy & birth and intake of antipyretic analgesics during pregnancy were the common risk factor for the first 5 birth defects.The key points for the prevention of birth defects are to reinforce the pre-pregnancy guidance,the screening and evaluation of risk factors and disease prevention during pregnancy.