Abstract: Objective: To investigate the effects of body mass index(BMI) in pre-pregnancy and different periods of pregnancy on gestational diabetes mellitus(GDM). Methods: The data of 2 260 singleton pregnancy women with complete prenatal records were retrospectively analyzed.The incidence rates of GDM between different BMI group in pre-pregnancy and different periods of pregnancy were compared. Results: The incidence rates of GDM in obese and overweight group during pre-pregnancy and early pregnancy were significantly higher than that in normal group and lean group(P<0.01).The incidence rate of GDM in obese group was higher than that in overweight group and normal group during mid-pregnancy(P<0.05).The difference of the incidence rate of GDM in different BMI groups during late pregnancy was not statistically significant(P>0.05). Conclusions: The overweight and obesity in pre-pregnancy,early pregnancy and mid-pregnancy can lead to the incidence increasing of GDM.The BMI in late pregnancy does not significantly affect the incidence of GDM,which may be related to the low body weight,insufficient normal body weight sample and intervention.