Objective: To study the value of fetal epicardial fat thickness (EFT) in gestational diabetes mellitus in the third trimester of pregnancy and its relationship with clinical parameters and perinatal outcomes. Methods: A total of 80 participants, including 40 with diagnosed GDM and 40 healthy pregnant women, were included in the study. Demographic data were obtained from medical records. Sonographic examinations were performed, such as amniotic fluid value, fetal biometric measurements, and Doppler parameters of the umbilical artery. Fetal EFT values were measured at the free wall of the right ventricle using a reference line with echocardiographic methods. Correlation tests were performed to evaluate the relationship between fetal EFT and clinical and perinatal parameters. P < 0.05 were interpreted as statistically significant. Results: The fetal EFT value was statistically higher in the GDM group than in the control group (p:0.000). Spearman correlation tests revealed statistically significant but weak positive correlations between fetal EFT value, 1-hour 100-gr OGTT, birth weight, and BMI (r: 0.198, p:0.047; r:0.395, p:0.012; r:0.360, p:0.042, respectively). The optimal fetal EFT threshold for predicting GDM disease was found as 1.55 mm, with a specificity of 74.4% and sensitivity of 75.0%. Statistically significant differences between the two groups in umbilical artery Doppler resistance index (RI), pulsatility index (PI), and systolic/diastolic ratio (S/D) were not found (p:0.337; p:0.503; p:0.155;). BMI and amniotic fluid volume were higher in the GDM group compared to the control group (p:0.009; p:0.000). Conclusion: This study demonstrated that increased fetal EFT may occur as a reflection of changes in glucose metabolism in intrauterine life. Future studies with larger series, including the study of neonatal metabolic parameters, will contribute to the understanding of the importance of fetal EFT in determining the metabolic status of the fetus.