Objective: To evaluate the changes of fetal epicardial fat thickness (EFT) in pregnancies with pregestational (PGDM) and gestational diabetes mellitus (GDM) and to identify the diagnostic effectiveness of fetal EFT in differentiating PGDM and GDM from normal pregnancies. Methods: The study was conducted with pregnant women who admitted to perinatology department between November 2020 and September 2022. Patients were analyzed as 3 groups: PGDM (n=161), GDM (n=171) and control group (n=170). EFT was measured in all three groups at 29 weeks of gestation. Demographic characteristics and ultrasonographic findings were recorded and compared. Results: The mean fetal EFT was significantly higher in PGDM (1.47±0.083 mm, p<0.001) and GDM (1.40±0.082 mm, p<0.001) groups compared to control group (1.19±0.049 mm) and was also significantly higher in PGDM group than GDM group (p<0.001). Fetal EFT was significantly positively correlated with maternal age, fasting, 1 ^st hour, 2 ^nd hour glucose values, HbA1c, fetal abdominal circumference and deepest vertical pocket of amniotic fluid (p<0.001). Fetal EFT value of 1.3 mm diagnosed PGDM patients with a sensitivity of 97.3% and a specificity of 98.2%. Fetal EFT value of 1.27 mm diagnosed GDM patients with a sensitivity of 94% and a specificity of 95%. Conclusions: Fetal EFT is greater in pregnancies with diabetes than in normal pregnancies, and also greater in PGDM than in GDM. In addition, fetal EFT is strongly correlated with maternal blood glucose levels in diabetic pregnancies.