Abstract: ObjectiveTo access the sex ratio at birth and its related factors in parts of China.MethodsUsing stratified random cluster sampling,we sampled one of cities from Inner Mongolia,Jiangsu,Gansu,Guizhou and Heilongjiang province,respectively,and investigated 17 093 infants born between 2006 and 2008 and their parents in the five cities from March through June 2009.ResultsFrom 2006 to 2008,the average sex ratio at birth was 119.48 (male to female) for the newborns during the 3-year period in the five cities and the ratio was different among the newborns of various regions,different live birth order,and different nationality (P<0.05 for all).The sex ratio at birth was the highest in Zhijin county (163.4) of Guizhou province and the lowest in Changzhou municipality (108.65) of Jiangsu province.For the newborns of different live birth order,the sex ratio was the lowest for the first-borns (112.63) and the highest for the third-borns (199.6);for the newborns of various ethnic groups,the sex ratio was the lowest for the newborns of Mongolian (106.01) and the highest for the newborns of Chuanqing nationality (176.41).The results of multivariate logistic regression showed that history of induced abortion (odds ratioOR=1.579),high live birth order (second born:OR=1.261,third born or above OR=1.395),boy preference of parents (father:OR=1.580,mother:OR=1.772),and low educational level of the father (<9 years:OR=1.244,9-12 years:OR=1.336) were associated with high sex ratio at birth and parental girl preference (father:OR=0.504,mother:OR=0.544) was related to low sex ratio.ConclusionThe factors associated with high male to female ratio at birth are history of induced abortion,high parity,boy preference in parents,and low education of father;the factor related to low sex ratio is parental girl preference in parts of China.