Objective. To evaluate the importance of determining fasting glycemia at the first prenatal visit for timely diagnostics of gestational diabetes mellitus (GDM). Material and methods. The study included 577 pregnant women examined with the use of the oral glucose tolerance test (OGTT) during weeks 24-28 of pregnancy. The results of the test were compared with the fasting glucose level at the first prenatal visit. Results. The mean fasting blood glucose level at the first prenatal visit was 4.70±0.61 mmol/l. In 25% of the patients, fasting glycemia at this time was 5.1 mmol/l or higher. Only in 36% of the women (52 of the 145 ones) with the fasting blood glucose level 5.1 and <7 mmol/l at the first prenatal visit, the results of OGTT were consistent with the criteria for GDM. The mean fasting blood glucose level at the first prenatal visit (up to week 16) was significantly higher than on weeks 24-28 (4.71±0.61 and 4.53±0.69 mmol/l respectively, p<0.001). The ROC analysis failed to reveal the fasting blood glucose level that could be used to predict the development of gestational diabetes with a reliable sensitivity and specificity. Conclusion. Further population-based investigations are needed to elucidate the relationship between fasting glycemia in the first trimester of pregnancy and its unfavourable outcome.