〔Abstract〕 Objective To investigate the effects of gestational diabetes mellitus(GDM) on birth outcome and umbilical artery(UA) blood flow parameters in the third trimester, and to analyze the role of UA blood flow parameters in GDM and birth outcome. Methods Based on the birth cohort from Wuhu, Anhui, China, 189 pregnant women with GDM were collected as the case group. The non-GDM pregnant women were matched 1 ∶1 according to age and pre-pregnancy body mass index, and 189 normal pregnant women were selected as the control group. Pregnant women with GDM were divided into poorly controlled group and well controlled group according to fasting blood glucose in the third trimester. The UA blood flow parameters and fetal birth outcomes in the third trimester were tracked. Results Compared with the control group, UA parameters in poorly controlled and well controlled groups significantly increased(F=6.63,P<0.05;F=4.43,P<0.05;F=5.57,P<0.05). Poor glycemic control of GDM was associated with increased birth weight and risk of larger than gestational age. The multi-factor linear regression model showed that the Z score of the peak systolic velocity/end diastolic velocity(S/D) in the poorly controlled group was negatively correlated with birth weight(β=-209.78, 95%CI:-301.48-118.07). S/D index Z score mediated the relationship between poor blood glucose control and birth weight. The intermediate effect value was-58.41(95%CI:-106.40～-19.65), accounting for 25.98% of the total effect. Conclusion Poor glycemic control in GDM is a risk factor for fetal weight gain, and UA function plays a partial mediating role in influencing neonatal birth weight. GDM pregnant women should strictly control blood glucose level to better protect maternal and infant health.