Purpose: The objective of this study was to estimate the accuracy of 22 fetal weight estimation formulas in diabetic pregnancies uncomplicated and complicated by fetal macrosomia. Methods: Retrospectively collected data of 317 pregnancies complicated by gestational diabetes mellitus and 78 cases of fetal macrosomia were used in this study. Study inclusion criteria were women diagnosed with gestational diabetes mellitus, full-term singleton pregnancy, and an interval from the ultrasound to delivery of ≤7 days. The estimated fetal weight was calculated using 22 formulas. The mean absolute percentage error (MAPE) and two-way random interclass correlation coefficient were chosen for statistical analysis. Results: In the group of gestational diabetes, MAPE ranged from 8.43 ± 10.17 to 54.01 ± 9.50%. Most of the formulas showed a tendency to estimate a lower fetal weight in comparison to the actual birth weight. In the group of fetal macrosomia, the correlations were poor. Only three formulas reached the threshold of MAPE <10%. Conclusions: The formula by Hsieh might be considered the best for fetal weight estimation in diabetic pregnancies. The combination of the best formulas might improve the accuracy of estimation. None of the formulas were accurate enough to predict fetal macrosomia.