Objective To access the sex ratio at birth and its related factors in parts of China. Methods Using 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. Results From 2006 to 2008,the average sex ratio at birth was 119. 48(male to female) for the new borns during the 3-year period in the five citie...